Ampeg GVT 52-112 to run EL34's

[MervsMods]

Would it be possible to modify my GVT to run EL34's instead of 6L6's? I've looked at schematics for both 6L6 and EL34 power sections, and in general, the main difference (that I see) is the resistors for 6L6 are 470 ohms and the EL34 are 1Kohms. (Both 5 watts)

Would a simple swap of those resistors allow me to run the EL34's? I've read and tried swapping these resistors in a first run Egnater Tweaker to allow EL34 operation with success.

How much "off" would this change be for the bias?

I've uploaded the schematic for the GVT. The resistors I'm wanting to swap are on page 2. R106 and R113.

Thanks in advance!

 

[Dacious]

It should be OK. It will likely sound pretty much the same.

 

[radiocaster]

You may want a lower second grid voltage on the EL34s like you say, but that would somewhat depend on the transformer.

The main difference is that the cathode and grid are not internally connected on an EL34. Or you can put in KT77s, which have that connection. In any case, you should bias the amp.

 

[MervsMods]

It should be OK. It will likely sound pretty much the same.

If that's the case, I really shouldn't bother then huh? I was just thinking out loud and thought I might have had an "a ha" moment, because I have a pair of NOS Mullard EL34's that I wanted to try out.

I guess I'm going to have to hunt for a JCM800 ...

 

[Wally]

As radiocaster notes, the suppressor grid and the cathode are not tied together internally in the EL34. One would have to assure oneself that pin 1 on the power tube sockets is open. If so, then one would have to tie pins 1&8 together on both tube sockets in order to run the EL34s. Too much trouble, imho. And....if I had a pair of matched real NOS Mullard EL34s, I would be looking for a better amp than a JCM 800 unless it was an early ‘80s Md. 2204, a Md. 1986, or a model 1987. Preferably, I would want an earlier Marshall for such tubes...a JMP or JTM...of those same models. Of course, the Md. 2204 did not exist in the JTM series as it was introduced in 1975 as part of the JMP series. Ommv.
As Dacious hints at, the circuit has more to do with the sound than does the power tube...not that there is not some difference.

 

[Dacious]

. And....if I had a pair of matched real NOS Mullard EL34s, I would be looking for a better amp than a JCM 800 unless it was an early ‘80s Md. 2204, a Md. 1986, or a model 1987. Preferably, I would want an earlier Marshall for such tubes...a JMP or JTM...of those same models. Of course, the Md. 2204 did not exist in the JTM series as it was introduced in 1975 as part of the JMP series. Ommv.
.

(Cough cough 4104)
I'd look for one of these - either version as the circuit didn't change.

I'd use them in my 4120, it's a sweet amp. All this series needs is a good Celestion like a G12-65, good biasing and good filter caps

 

[Wally]

(Cough cough 4104)
I'd look for one of these - either version as the circuit didn't change.

I'd use them in my 4120, it's a sweet amp. All this series needs is a good Celestion like a G12-65, good biasing and good filter caps
View attachment 752867

I just recapped a very clean and original 2205....head version of the 4210 combo above. I did not care for the amp. It had a good clean channel and decent reverb, but the distortion mode was not what I would want to use. I hear the diode clipping circuit, I suppose, and don’t care for it.
A little 4104 combo version of the 2204?? I would like to hear one.

 

[Dacious]

I just recapped a very clean and original 2205....head version of the 4210 combo above. I did not care for the amp. It had a good clean channel and decent reverb, but the distortion mode was not what I would want to use. I hear the diode clipping circuit, I suppose, and don’t care for it.
A little 4104 combo version of the 2204?? I would like to hear one.

Is this the 85+ chip-switching version, or the earlier one with 3-diode switching, and 6.3 volt 3000uf cap to earth out the dirt when switched to clean?

The 86-88 version supposedly sounds a lot different to the earlier one I have. Less gain on tap.

I don't mind the dirt channel - but honestly, I don't use it. If you play the gain, level and eqs good 70-80s tones are there. You can also set it up for a dark sort of Indie punk clean like a 5150.

Where the magic is, is the clean. It's got probably the nicest reverb not in a Fender. Of the two I had the one I sold with the Accutronics tank was fuller, but I don't use much verb so it's got the no-name tank.

The gain, and Baxandall style tone stack work very well. You can go from a lovely clean with gain and treble bass backed to JCM800 goodness all through. The post-phase -nverter master volume is possibly the most transparent design. You can dial in nice grind at any volume and crank to desired volume without having to tweak.

You can get a great dirty sweet woody Marshall tone and that nice touch sensitivity like a Princeton or the Superchamp.

I decided to keep the one that's got the Emi CV75 speaker and Svet tubes from the old factory and it's very usable. I updated the circuit as we discussed on the other thread and it's picked up punch and volume. If you move the feedback resistor there's also improvement to be found.

 

[Wally]

Dacious, this one was from 1986.
https://schematicheaven.net/marshallamps/jcm800_splitch_rev_50w_4210.pdf

I liked the reverb and the clean channel. It seems we agree on the gain channel...not usable for me.

 

[MervsMods]

Where do you guys find these amps??? I'm soooooo jealous!!