Grommes 240 conversion to TCJ EL84 (need help)

[bobafet]

Hi all!

Grommes model 240. The schematic is attached.
I bought it because I liked the look.))
I replacemetnt all electrolytes caps and listened for a while. In my opinion, the sound not the most outstanding. But it was not bad.

But something went wrong. Not so long ago, one of the el84 tubes failed. (tube glowing blue). I replaced the quartet el84 and after a while it happened again.

Without becoming to find out what the problem is (probably in vain), I simply dismantled the entire amplifier and decided to restore the chassis first.

Now I thought, maybe it's better to change the circuit to something with more affordable tubes?

The reasons:

• 7199 (6an8) and 6973 are not the cheapest and most affordable.
• Plates el84 run at 425vdc. (Maybe that's why the tubes quickly fail.)
• I can't find quality 500k potentiometers
• PT is heating up. Many tubes with high current consumption.

So far I have settled on EL84 PP from TubeCad. With the replacement of 6N1P with E88CC.
One of the few tested circuits + not low voltage.

What's available.
OT - 9000 - Primary (no UL)
PT - 350-0-350 (200mA) + 3.15-0-3.15 (6A)
Rectifier - pair ez81 or CC (octal tube will not fit in the chassis)

Now I'm struggling with the power scheme.
With my beginner knowledge I can't find a neat way to get close to 340V from 700CT.

I have a choke LL1685 10H / 160mA / 130r (saturating current 230mA) But I'm not entirely sure that it will work as a LC filter with reg.capacitor. Without an choke input , i need a lot of dissipation in the resistors 800R 25W .

I would really appreciate any advice on a power supply circuit or an amplifier circuit.

 

[Kay Pirinha]

I would have replaced the coupling caps C8, C9, C12, C13 first before dismantling the unit.

Best regards!

 

[Francois G]

Your Grommes 240 is a fine instrument with sophisticated (for its amp class) screen regulation with the (now) expensive 6973. They did run the EL84 hard, perhaps too for hard expensive-getting EL84s as aged NOS or modern manufactured tubes. However, I would suggest a different approach than you are contemplating.

I would keep the power supply near stock with the following modifications:
1. If your 6973 is tired, consider replacing with a 6CZ5. At the B+ to Vg2 drop a 6CZ5 should work well. (Watch for pin 8 is different in base diagram) 6CZ5 costs much less than 6973, and in this application there are many such tubes (6CW5, etc.) that will work, although some tweeking might be required.
2. Reduce the EL84 currents by making the the bias more negative. You could simply increase the cathode resistor values. Or to improve the amplifier I personally will use Dave Gillespie’s EFB(tm) biasing scheme. With EFT you can reduce the quiescent current through the EL84s by ~25-35 % AND get more power at lower distortion. I have used this on a few EL84 amplifiers with excellent results, using power tubes more sparingly and reduce waste heat. Read the article on his store about the DYNACO SCA35 EFT modification. http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf

He has a nice PCB custom made for the SCA35. http://www.tronola.com/html/daves_store.html Perhaps you could make it fit the Grommes, or point wire it yourself.

 

[Francois G]

I meant to add that instead of the 6973 tube it would be easy and inexpensive to employ a mostfet to control the voltage. Even better, Dave Gillespie’s mods routinely engages this approach (on pentode outputs) in his EFBII scheme to control the screen voltages in a fixed ratio with B+, rather than a fixed, regulated Vg2. More information is here covering a Fisher AA-100, which is similar to your Grommes with EL84/7189 outputs:
https://www.audiokarma.org/forums/i...-pics-fisher-sa-100-clone-in-progress.466229/

Here is his design for Fisher 400, which uses 7868 outputs, but the voltages are similar to your application with EL84 and could be tweeked. Higher that datasheet voltages for EL84 were originally used by Grommes, but by keeping the plate dissipation below 12watts per EL84 with EFB, they will live happily at these voltages:

 

[Chris Hornbeck]

I would say that your proposed power supply scheme is excellent. B+ voltage can be easily adjusted with the value of the input capacitor (more or less than 3u3F). When this amp was designed, valves were cheap and available, so were commonly abused to give more saleable WATTS. Today's perspective is different.

Similarly, the output stages' common cathode resistor and tiny bypass capacitor are cost-saving artifacts of their time, and should be corrected. Individual cathode resistors with much, much larger bypass capacitors give superior performance, and are easily affordable today.

All good fortune,
Chris

 

[bobafet]

Kay Pirinha​

These caps have already been replaced by the previous owner. Electrolytes were also new, but they were noname and with overestimated capacity.

Francois G​

Thanks for your recommendation, but I already completely disassembled the amplifier and sent the chassis for powder coating, I hope the color will be close to the original. + After checking some of the resistors, their ratings drift a lot and you need to buy new ones. For the assembly of TCJ EL84, I have almost all the denominations, so it will be easier. Perhaps after a while I will return to the original scheme on your recommendations.

Chris Hornbeck​

Thanks for rating! The book by Morgan Jones and this forum helped me a lot.

As for cathode resistors, yesterday I read an article by Dave Gillespie recommended by François G.
Additionally, a short search on the forum led me to a circuit with LM317 (the picture is attached). I'll try to implement this because the reviews are positive.


Thanks everyone.

 

[Steve Morley]

I recently rebuilt two Fisher 20-A mono amps. The original circuit was pretty crap, no grid stoppers, super-high B+, over 100% dissipation on the EL84s. I changed it over to the Tubelab SPP circuit (12AT7 gain/phase splitter) and added a couple of extra nodes in the B+ with a big resistor to drop the voltage down to around about 325. I cleaned up the grounding and floated the filament supply like the SPP does. They work great and don't eat their tubes. If you are going to run triode mode, you might want to really watch your B+.

S.

 

[Kay Pirinha]

Ok, then I have no idea about the EL84's blue glow always at the same spot.
Best regards!