Mod Sovtek/Marshall EL34 to EL84???

[chipaudette]

Hi All,

I admit it...I grew up on pre-amp distortion. But, 6 months ago, I bought a Fender Deluxe Reverb Reissue and I'm really liking what I hear from power amp distortion. I'm also having fun modding the DRRI to various evil ends.

Now, though, I'd like to get a taste of some non-Fender power tube overdrive. So, I been over to Ebay and have been tracking some 50 Watt Sovtek heads for the last two months or so. They look like basic Marshall circuits. I like that. But, I don't think that my hearing can take 50 Watts. It's just too much power.

So, now I'm thinking that maybe I can buy one of those heads and then mod the power section to use lower power tubes. You know, like swap out the EL34s for EL84s (or something else). This would certainly lower the power output and volume. What kind of mods are necessary to make this happen?

Yes, I do have a THD hotplate, but even for the Fender I'm using up most of its attenuation when I open up the amp to good saturation. If I went to a 50 watt amp, I don't think the hotplate would be enough. That's why I'm looking to mod the amp.

What do you think?

Chip

[Zoe_N_Iain]

Yeah, it's a tough one to get good power valve crunch without the ear-crushing volume that goes with it.

To be honest there isn't a massive difference in volume between 50W and the ~18W you'd get with EL84s. You'd need to get down to 5W to actually be 1/2 as loud through the same speakers.

In the case of the MIG 50, you also have the problem that the PA voltage is too high to run EL84s in place of the '34s. You'd need to use a pair of Yellowjacket adapters or similar.

Another option might be to buy/build a less efficient speaker cab. Lowering the sensitivity of the speakers by 3dB will halve the volume. Say your current one is 100dB/W/m and you find another that's 97, it will be half as loud. 94 will be half the volume of that, and so on.

[chipaudette]

Hi,

The Yellow Jackets are a cool option that I didn't know about. Thanks! But, they are a touch expensive, especially if I can get away with just a little re-wiring and a couple of resistors or caps. So, I've got two follow-up questions:

(1) What are typical plate voltages for EL34 vs 84? Could I step down the voltage with a power resistor?

(2) As for 50W vs 18W being not much volume change...my understanding is that, when in power limited operation (like when I'm overdriving the power section), the volume changes with power according to 10*log10(P2 / P1). In our case, this would be 10*log10(18W / 50W) = -4.4 dB. Have I misunderstood things?

There are two reasons why I think that 4dB would be worth the effort. First, 4 dB is one notch on my Hotplate. I find that one notch on my hotplate is a fairly substantial change in volume. Second, my Deluxe Reverb is rated at 22W and I can deal with it. If I could get the Sovtek from 50W down into the 20W territory, then I've turned it into a beast that I can live with. I like that.

Finally, I like your thoughts on reducing the efficiency of the speaker cab. That would be a great way to reduce the volume. But, like the Yellow Jacket solution, buying a lower efficiency speaker (or building a lower efficiency cabinet) could cost a bit of money. A bit of re-wiring sounds cheaper. Of course, it just might not be a reasonable thing to do.

Chip

[Amp Kat]

The main drawback here is that your OT may not take the extra wattage especially at power tube distortion. If you can get a beefier or higher wattge transformer you would be much better off which isn't a huge deal but you would most likely have to hack it to make it fit. By the time your done you may as well find a ebay EL-34 amp for way cheaper.

[Bruce / Mission Amps]

Originally Posted by chipaudette

Hi All,

I admit it...I grew up on pre-amp distortion. But, 6 months ago, I bought a Fender Deluxe Reverb Reissue and I'm really liking what I hear from power amp distortion. I'm also having fun modding the DRRI to various evil ends.

Now, though, I'd like to get a taste of some non-Fender power tube overdrive. So, I been over to Ebay and have been tracking some 50 Watt Sovtek heads for the last two months or so. They look like basic Marshall circuits. I like that. But, I don't think that my hearing can take 50 Watts. It's just too much power.

So, now I'm thinking that maybe I can buy one of those heads and then mod the power section to use lower power tubes. You know, like swap out the EL34s for EL84s (or something else). This would certainly lower the power output and volume. What kind of mods are necessary to make this happen?

Yes, I do have a THD hotplate, but even for the Fender I'm using up most of its attenuation when I open up the amp to good saturation. If I went to a 50 watt amp, I don't think the hotplate would be enough. That's why I'm looking to mod the amp.

What do you think?

Chip

Nope... most of those old Mig50s have a B+ rail that is OVER 500VDC!!! I don't think there is a 12-14 watt power tube made that could survive that, even if you cathode biased them.
Of course you could get rid of the power transformer and use a smaller one... or use a 50w reverse biased zener diode in the B+ rail to cut some of the high voltage.
I think this would be a challenge.

[Zoe_N_Iain]

A JJ 6V6 probably would, provided you drag down the screen voltage a lot. But again, that's likely to be a pain in the arse to do.

[Amp Kat]

Oops. I read your initial post wrong. I thought you wanted to go EL-84 to EL-34 on the Fender DRR which wouldn't have worked well either. My bad ,but I do agree with the rest that it would be hard going EL-34 to 84. The Sovtek MIG is one of the loudess and stout amps I've worked on. Very underated and sounds good biased a little hot and using the MV at low levels. That was actually the amp that got me put in the dog house when the little lady next door came over and said it was way to loud. I'm a big NO with 84's also.