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UPGRADES AND MODIFICATIONS FOR DENTRON GLA-1000 SERIES AMPLIFIERS

There are some improvements you can do that will protect your meter and tubes:

1. Install a 3 watt 10 ohm resistor MOF (metal oxide film type) between the cathode pin and the cathode circuit. This requires you to cut the cathode trace (Pin #3) and bridge across it with the resistor.  **If you have done the Svetlana modification\conversion to the EL-509\6KG6 tube Pin#9 is the cathode and this is where you will have to install the resistor (bridge across a cut in the trace).** This resistor helps to keep the cathode current balanced between the tubes and improves linearity. In the event you loose cathode bias it will also limit cathode current and not let a tube or tubes run away and blow. In this case the resistor may open like a fuse!!

**Take note that there is some difference in the Pin out between the GLA-1000 and the GLA-1000B (the way the pins are soldered on the bottom of the tube socket).

GLA-1000 (Pin #1&2 are connected and on the GLA-1000B Pin#2&3 are connected.  (In the case of the GLA-1000B the Svetlana conversion requires you to jumper pin#1 to Pin #2&#3) and if you have a GLA-1000 amplifier you will have to jumper Pin #3 to Pin #1&#2.

 2. Install a 25 ohm 10 watt vitreous (glass type) resistor between the RF Plate Choke (RFC 6) and the +HV power. On the bottom of the amp you will see a red (or some other color) wire running from the bottom of RFC 6 to the +HV side of the rectifier diodes. Cut the wire and install the resistor between the cuts in the wire. Isolate the resistor and connections from the chassis so you don't get a short.  I glued a piece of perf board down the bottom of the chassis with high temp red RTV and lightly glue (same red rtv) the resistor to the perf board.  This isolates the resistor from the amp's chassis and gives you some room clear of the chassis for the terminations.  This resistor will protect the tubes and power supply from any HV shorts to the chassis.

 3. Install one 1N5408 diode across each electrolytic capacitor in the power supply. These are for reverse current protection. (Banded end of diode to + terminal on capacitor)

4. Install two series connected 1N5404 diodes (cathode to anode) in parallel with R7 (meter shunt). These diodes will protect the meter from a HV glitch. Solder the cathodes (banded ends) of the diode string at the point where R7 is soldered to the negative terminal of C23 (the anode of the diode string is soldered to ground).

5. Consider replacing all the diodes and electrolytic capacitors in the power supply if they are old. Sometimes a lot damage is incurred when they fail. Diodes are 1N5408 and the 500 vdc 100 mfd capacitors can be found at Newark Electronics-Newark Part Number 95F4424.  I use the Newark 220 mfd 500 volt capacitors  (Newark part# 95F4425)      OEM (Part# 82DA221M500KE2D) in my amps to increase the capacitance.  These new Newark capacitors are the radial snap in type. UPDATED ***You can also use CDE type 380LX capacitors available from Mike at mikeyman@rochester.rr.com.  Mike's price is the best I have found so far.  Straighten out the leads a little and mount them as you would any radial leaded capacitor.  You will have to use insulated wire jumpers to make some of the terminations.  The old can type capacitors had more than one negative terminal and the new ones only have one.   In addition it is very important you make sure you have all the 100k ohm 3 watt voltage equalizing resistors for the capacitors connected.  The removal of the old can type capacitors eliminates a few of the connection points for the resistors so you will have to install a couple of jumpers.  All 3 of these capacitors are wired in series.  Just remember to go positive to negative for each capacitor.  

 6. Install a 75K ohm resistor in the cathode circuit to cut off the tubes when in stand-by or receive. I used a 3PDT relay in my amp so I could use the spare set of contacts for the cathode bias and tube cut off. The resistor is in series with the cathode bias during stand-by and receives and out during transmits. This simple mod will increase the life of your tubes greatly!! See attachment!! Unzip the Dentron files at Skipp May's web site and find some good Dentron amp info!!   http://sonic.ucdavis.edu/so02000.html

7. Install a 1N4007 diode across the T/R relay coil. Banded end of diode to +DC.

 8. Install inrush limiters or a step-start relay. Look here- http://www.megasurge.com/Bigamp_datasheet.htm

 I used their Part number- SL32 10015 (10.0 ohm 15.0 maximum continuous amps)

 9. Install two MOV (Metal Oxide Varistors) one each across the hot and neutral ac power wires to ground. AC Power glitch protection!!

10. Check the voltage of the secondary of the Filament winding of your transformer. Do not exceed the maximum filament voltage rating of the tubes of about 6.8 vac (No matter what tubes you are using).  I recommend a Filament voltage of 6.3 to 6.5 vac.   If you have to, install a resistor in the secondary of the filament transformer to reduce the voltage.  R=E/I

11. Svetlana Dentron GLA-1000 tube conversión info. It works for all the GLA-1000 series amps. Look here. http://www.svetlana.com/docs/TechBulletins/technoteNO.32.html I have done several of these conversions and they work great!!

If you do the Svetlana conversion take notice that the Cathode Bias voltage requirements will be different than that of the amp's original tubes (6LQ6).  The GLA-1000 used a 9 vdc zener diode and the GLA-1000B used a 24 vdc zener diode for cathode bias.  You are looking for about 100ma idling plate current, current when the amp is keyed with no drive applied from the exciter.  For my EL-509/6KG6 conversions I have found the cathode bias produced from a string of eight 1N4007 diodes is just about right.  I used series connected 1N4007 diodes to establish the correct bias voltage instead of buying a zener (over $14.00 each now).  Eight 1N4007 diodes series connected gave me the +vdc required to establish the 100ma idling plate current.  I mounted them on a piece of perf board and suspended them off of the T/R relay cathode bias contact.  I have also mounted them on the bottom of the amp underneath the transformer to open up the area around the T/R relay.  This is now my preference.  NOTE GLA-1000 owners:  The small circuit board hanging off the back of the chassis (facing the amp it is just to the right of the T/R relay) is the Relative Power circuit.

12. Also consider installing an insulated board underneath the top cover where the top of the tubes comes so close to the bottom of the top cover!! (5" x 5 1/2" piece works well). Check Mc Master Carr for Muscovite Mica Sheets on page 3190 of their catalog. Part Number 8802K19. This is a 5 x 8 inch rectangle piece. Cut to 5 x 5 1/2 inches and it will fit perfectly. Glue it in with High Temperature red rtv then when dry flip the cover over laying it on a 2 x 4 or equivalent and while using the original vent holes in the cover as guides, drill vent holes in the insulating material.  This stuff is hard so use a carbide bit and take your time!!

This material is .004 of an inch thick and has insulating strength of 600 volts per mil of thickness.  So a rectangle of this is good for up to 2400 volts.

13.  The Parasitic Suppressors should also be replaced, especially if they are made up with Carbon Resistors.  Look here for Parasitic Suppressor kits:

http://www.vcnet.com/measures/Price-Info.html

The kits made for the 811A and 572B tubes work fine for the 6LQ6 or the Svetlana EL-509/6KG6 tubes.  You can also go with 4 turns of #16 bare copper wire in parallel with two 100 ohm 3 watt Metal Oxide Resistors (MOF).  Don’t let the resistors touch the coil.  Wind the 4 turn coils around the shank of a 3/8 inch drill bit.

14.  If you choose not to use Inrush Limters and prefer a Step Start relay look here!!

Step-Start Instead of Inrush Limiters

Most power supplies benefit from something to soften the shock of start-up. A 10A DPST-NO or 10A DPDT relay and two approximately 25 ohm 10W resistors are just about all that's needed to add a step-start circuit to the average 1500W amplifier. The step-start circuit goes in series with the mains fuses or circuit breakers. With this arrangement the filaments, the HV supply and the LV supplies enjoy the benefit of a kinder and gentler start-up.

Diagram here:

http://www.vcnet.com/measures/D.a.01.GIF

 Find best price on the EL-509/6KG6 tubes here ($14.00 each with a minimum of 5 tubes)!!

http://www.hyelectronics.com/

2nd best price here $14.50 each

http://kwtubes.s5.com

Next best price for tubes (Svetlana EL-509\6KG6)-

$24.99 each

http://www.tubeman.com/itm00113.htm

Russian Number for the

Svetlana EL-509\6KG6 is:

6pi45C (6P45S)

FAR Circuits is now offering a 3 board set (RF Board, Power Supply Board, and Tuned Input Board) for the Dentron GLA-1000 series amplifiers. They are $23.00 a set plus $2.00 shipping. I have no connection to FAR Circuits, just passing along some good to know parts info. The Tuned Input Board offered is the design offered in the GLA-1000B amplifier. I intend to use one of these boards to add a Tuned Input to my GLA-1000.
FAR Circuits Web Site:
http://www.cl.ais.net/farcir/

FAR Circuits is now going to offer up an RF Board (tube board) for the Dentron GLA-1000 series amplifiers that is made for the Svetlana EL-509/6KG6 tube sockets. I sent him a new Svetlana tube socket for a pattern. I have several more of these amps awaiting conversion to the Svetlana EL-509/6KG6 tubes and didn't want to have to use the plastic (uck) 6LQ6 tube sockets in the amp. The Svetlana EL-509 tube sockets are ceramic and appear very well made. The orginial 6LQ6 tube sockets can be used if the holes are opened up a little but I have had problems with breakage and loose connections with the tube pins.

FAR Circuits is also going to re-route the circuit board trace on the RF Boards to accommodate the pin out differences between the 6LQ6 tubes and the Svetlana EL-509/6KG6 tubes and is adding trace and holes for the addition of the Cathode Resistor.  In addition they are redesigning the Power Supply board to use radial lead type electrolytic type capacitors instead of the old original can type.  The old can type used on the Dentron amps had 4 negative terminal connections of which only two where used in the circuit.  This required you installing a couple of wire jumpers on the old Power Supply board if you changed over to the radial type capacitors.   The new and improved Power Supply Board offered by FAR Circuits has the circuit board trace made up to accommodate the conversion to radial lead type capacitors without having to use wire jumpers.  The conversion to the Svetlana tubes is now too easy to pass up.

I have discovered that the only real way to tune one

 of these amps is with a Tuning Pulser.   
The key down approach while very popular is not the best way.  The

 first thing wrong with 
that method is that the Plate

 Voltage drops significantly during the tuning and it

 is hard on
 the tubes.  Remember that the Dentron

 GLA-1000 series of amp's power supply is fine for 
SSB

 but going key down will stress it a lot.  It will drop

 your Plate Voltage enough that you 
are not getting the

 most optimum tune up for SSB.  "FAR Circuits has

 agreed to provide a 
Circuit Board for the Tuning

 Pulsers for those of us who choose to build our own."

 

Tuning Pulser Schematic here and Tuning Procedure:

http://www.vcnet.com/measures/D.a.09.GIF

 

C8- A 500pf 6kv ceramic capacitor.  This cap is installed in parallel with the Rf Choke (RFC-6).  
It's purpose is to bypass to chassis ground any RF that get's past the RF Choke.  I suggest you replace it.  
I have found several that had failed.  I suggest you use a 6kv 2000pf capacitor instead. 

 

Plate Choke (RFC-6)-   Prevents RF from getting into the Power Supply 

Don't be surprised if you have to add or remove a turn or two from the Plate Choke in order to get full power out. 
 This is found mostly on 10 and 15 meters.  Removing turns increases the Plate Choke's resonant frequency and adding 
turns lowers it.   It is imperative that it does not resonate in any part of the Ham Bands or it will burn up and may let RF back 
into the power supply which could cause damage.

 

From Rich Measures (AG6K) web site (http://www.vcnet.com/measures/):

Anode HV RF Chokes

The basic requirements are: 1. The choke must have ample reactance at the lowest operating frequency to limit the RF current through the choke to a reasonable amount. 2. The choke can not be self-resonant near an operating frequency. 3. The wire gauge used must be able to carry the DC anode current plus the RF current at the lowest operating frequency without excessive heating.

If the HV-RFC has a self-resonance on or near an operating frequency, potentials of many times the anode supply voltage can appear on the choke. When this occurs, a choke arc and fire is likely. Choke fires can destroy more than just the choke because the rising plume of ionized gasses from the choke fire often creates a conduction path to the ceiling of the RF output compartment. If an arc occurs, pervasive damage is likely if no glitch protection resistor was used in the HV positive circuit.

Choke Design

Materials:
There are two types of wire insulation materials that are suitable for use in HV-RFCs--silicone varnish and Teflon. Modern, high-temperature electric motor wire is insulated with a tough, silicone varnish that can handle high DC voltage and high RF voltage. At room temperature, a twisted pair of #20 silicone varnished wires can withstand more than 5000 VDC or 1500W in a 50 ohm circuit at 29MHz. This type of wire is sold by the pound in electric motor rewinding shops. If you want to buy some, bring your own empty spools and winding device--such as a variable-speed electric drill, with a homemade adapter to hold the spool. Due to its toughness, silicone varnish insulation requires a special method of stripping. An open flame from a butane lighter causes the silicone varnish to decompose and combust. The remaining ash residue can be removed from the copper with steel wool.

Teflon insulated magnet wire is not common. Although ordinary Teflon insulated hookup wire may be used, the extra insulation thickness requires that a longer coil form be used. One potential trade-off with Teflon insulated wire is phosgene. When Teflon burns, deadly phosgene [COC2] gas is produced.

Due to contact with air, the current carrying ability of either type of wire is much higher in an HV-RFC than it would be in a transformer. #28 wire will easily carry 1A in a HV-RFC. #24 will carry several amperes with acceptable heating.

G10 or G11 epoxy-fiberglass tubing is RF-resistant, strong, and easy to work with. It is an ideal material for building HV RF chokes. It can be obtained from plastic supply houses. 1mm wall thickness is more than adequate. Diameters of 16 to 25 mm are typically used for building HV-RFCs. G10 tubing can be cemented to a G10 base plate with silicone rubber adhesive or epoxy. A source of G10 tubing: Plastifab,1425 Palomares, La Verne, CA 91750 818 967 9376.

It is probably a good idea to limit RF current in the HV-RFC to no more than 1 ampere. To calculate current in the choke, take roughly 2/3 of the anode supply volts and divide it by the reactance in ohms at the lowest operating frequency -- a.k.a. Ohm's Law.

Determining Bypass C

Power supply components can be damaged by RF. Electrolytic filter capacitors are especially at risk. Thus, adequate RF bypassing on the power supply side of the HV-RFC is needed. Probably no more than 10V of RF should be allowed to appear on the +HV supply at the lowest operating frequency. Determining just how much bypass C is needed basically involves using ohm's Law. The amount of RF current flowing through the choke and the amount of bypass C need to be evaluated for the lowest operating frequency--usually 1.8MHz. For example, if the reactance of the choke is +j2000 ohms, and the AC anode voltage is 2000Vrms, then I=2000V/2000 ohm=1A of RF flows through the choke. In order to limit the RF voltage to 10V maximum at 1.8MHz, 10V/1A=10 ohm of capacitive reactance is needed for an adequate bypass. Using C=1/(Xc * 2pi * f), this equates to a HV bypass capacitance of 8842pF. Obviously, a typical 1000pF bypass C [minus j88 ohm] is not going to do the job because it would allow approx. 88V of RF to appear across the HV supply if 1A were flowing through the choke.

500pF 20kV TV-type doorknob capacitors are NOT designed to handle RF current--so they do not make satisfactory HV bypass capacitors. Disk ceramic capacitors may be used for HV bypassing. Disk ceramic capacitors are somewhat limited in the amount of RF current they can safely handle. Manufacturers typically don't publish RF current ratings for them. To find out how different capacitors react to RF current, you must test them yourself. Even a 7500WVDC, 2500pF disk ceramic capacitor becomes warm from 1A at 1.8MHz. Thus, it is often best to parallel a number of individual bypass capacitors so that the RF current will be shared among them.

Determining L

At the lowest operating frequency, the HV-RFC should have enough reactance to limit the RF circulating current through the choke to a reasonable amount. Allowing a RF current of 1A RMS through the choke usually does not create problems for the wire-lead disc-ceramic capacitors that are typically used to bypass RF on the power supply side of the HV-RFC. To minimize RF current through the choke, it would seem that more inductance is the answer. However, more inductance means more choke resonances and a greater likelihood of choke fires. A compromise is indicated.

Over the years, various schemes have been used to minimize choke resonances. Adding gaps at presumably esoteric positions in the winding was represented as a means of decoupling parts of the choke winding--allegedly ameliorating the self-resonance problem. However, when the resonances of gapped chokes are compared to similar chokes without gaps, no real improvement is observed on a dip meter. This should not be surprising. Optimum decoupling between two coils occurs when they are mounted at a right angle. Adding end-to-end spacing with gaps is the least effective decoupling method possible. To minimize resonance problems, instead of using a single large choke, use two smaller chokes mounted at right angles.

The highest-L choke that can built that is free of self-resonances in the HF spectrum is roughly 60µH. At 1.8MHz, 60µH has a reactance of about +j679 ohm.

The RMS voltage that appears across an amplifier's HV-RFC is approximately two-thirds of the anode supply voltage. For example, an amplifier that is powered by a 3000V supply subjects its HV-RFC to about 2000V RMS. If a 60µH inductor was used in this amplifier, at 1.8MHz the RF current through the choke would be 2000V/679 ohm=2.95A RMS. Adequately bypassing approx. 3A of current on the power supply side of the choke is difficult. A typical HV disk ceramic bypass capacitor can handle only about 1A. Another problem is that at 1.8MHz 130pF [minus j679 ohm] of extra capacitance is required from the tune capacitor to cancel the +679 ohms of reactance in the choke. Adequately bypassing 3A at 1.8MHz requires a substantial amount of capacitance. To hold the voltage across the bypass capacitors to less than 10V at 1.8MHz, roughly 0.026µF [minus j3.3 ohm] is indicated. To handle this amount of current, four approx. 0.0075µF HV disc ceramic capacitors would probably be needed. All things considered, using more inductance is indicated. Limiting the HV-RFC's RF current to a maximum of !A would make the task of bypassing a lot easier. However, increasing the inductance above 60µH is virtually certain to move choke resonances into the HF range. Unless these resonances are prudently parked between operating frequencies, a choke fire may result.

To realistically evaluate the self-resonance situation, HV-RFCs should be checked with a dip meter after they are installed and wired in the amplifier. If a self-resonance is within about 5% of an operating frequency, there may be a problem. When re-parking resonances, it is usually best to remove turns from the choke. This will move the resonances up in frequency--and only slightly increase the maximum RF current through the choke.

In continuous coverage amplifiers, there are obviously no safe parking places for choke resonances. The only solution is to switch HV-RFCs with one or more HV vacuum relays.

HV-RFCs should be single-layer solenoid wound. To minimize wire vibration during operation, the wire should be under constant tension when winding and soldering the ends to the solder lugs. When silicone varnish insulated wire is used to wind a HV-RFC, the finished winding should be given a coat of gloss urethane varnish to hold the wire in place. Since varnish will not adhere to Teflon wire, a different method is needed to keep a Teflon winding taught. Small tensioning springs are soldered to the ends of the wire. The springs provide constant pull to minimize wire vibration during modulation. An S-shaped copper foil jumper should be connected across each tensioning spring.

DC Blocking Capacitors

Blocking high voltage DC is the least difficult part of the blocking capacitor's job. During operation on 10m, the DC blocking capacitor must be able to carry most of the RF circulating current in the tank. Here's why: The amplifier tube's anode capacitance normally provides most of the tune capacitance during 10m operation. Thus, a major portion of the tank circulating current passes through the anode capacitance and therefore through the DC blocking capacitor. In an amateur radio amplifier, blocking capacitor currents of 5 to 10 A RMS are not uncommon during operation on the 10m band.

Selection of a blocking capacitor should not be guesswork. It is advisable to select a capacitor or capacitors that is rated to carry the calculated maximum RF current present. Merely selecting an RF-type (transmitting) capacitor is not good enough. Some RF-type capacitors have rather unspectacular current capabilities. The capacitance of the DC blocking capacitor is not very critical. 1000pF seems to be more than adequate for operation at 1.8MHz. 88 ohms of Xc is relatively insigificant in comparison to the typical 1000 to 2000 ohm anode output Z.

 

This is an alternate modification that can be done instead of the Svetlana modification.  It is a circuit by (PA0FRI) Frits Geerligs  Email- pa0fri@amsat.org

 

Schematic and notes here:  Frits uses EL-519 tubes but they are very similiar to the EL-509 so the design can be used as is.

 

http://hem.passagen.se/sm0vpo/tx/500w-txt.htm

 

I can provide more information if needed as I have identified all the components and found sources for them.

73,

Bill Smith KO4NR