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Which preamp to match 2xEL84 class A ?

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9/8/1997 10:00 AM

Which preamp to match 2xEL84 class A ?

Hello,

Last month I've finished my DIY project (3xECC83, 2xEL84)
Power amp: Class A cathode biased 2xEL84 power amp with a cathodyene (1/2 ECC83) phase splitter. Circuit copied from a Crate VC2110.
Preamp: Tweed Fender Bassman cathode follower preamp. The output level of the preamp was too high, I reduced the output by changinging the anode/cathode resistors and a voltage divider after the tone stack.
Tremolo: Single stage (1/2 ECC83), copied from a Gibson Hawk.

This amp sounded fine, especially with my Telecaster. You can get a really good blues sound with just a little tremolo and the power amp cranked halfway.

I'd like to built another amp like this, but with a different preamp. I would like to get a really bright old Fender tone, with lots of compression when pushed up. The amp I've built sounds cool played semi-clean but pushed up the sound is al little too flat and bassy and I think it's beacause of the preamp.

Can anybody suggest a nice preamp? I was thinking about a Blackface preamp. Does anybody have experiece with this combination?

What is your opinion on single (cathodyene) and double stage (VOX, Marshall) phase splitters ? I tend to think the single stage splitter lacks headroom. But I guess I don't think I really need the amplification of a double stage splitter in a 2xEL84 setup.

I'd also like to build a tremolo for this amp. I was thinking of copying the trem from the Tremolux with EL84's. Does anybody have experiece with this combination/amp?

Thanks, Niek (The Netherlands)

9/10/1997 8:36 PM

Hey Niek,

Congrats on your first amp! My DIY experience has been limited to modding an old Marshall, it had been modified before I got it so I wasn't ruining a classic. Anyway, if the preamp in your amp isn't bright enough for you, you might consider using smaller coupling capacitors. I don't have a schematic in front of me but I'm pretty sure that the AC-30's used 500pF coupling caps where the Fender's used .047uF, quite a difference. This will block more of the low end giving your amp a brighter sound. A lot of the old Fender tone came from the 6L6 output tubes which are alot harder and brighter than the EL84's. Your description "a really bright old Fender tone, with lots of compression when pushed up" sounds like you might want to try the Vox AC-30 Top Boost preamp circuit for your next project.

Good Luck, have fun, and watch those high B+'s,
Roger

9/11/1997 1:57 PM

Thanks Roger

Hi Roger,
Thank you for your info. I already tried some other coupling capacitor values. I haven't tried extreme values like 500pF in my Bassmann preamp, I think that might be a little too bright. Since the amp sounds cool halfway, I'll try to lower the cathode bypas resistor of the EL84's. The current capacitor value is 220uF, I'll try 100, 47 and 25 uF. Unfortunately I have to go abroad for five months so you'll have to wait if you want to hear my experiences. I can however still stay in touch, hotmail is worldwide accessible.

Where do you US guys buy your parts and transformers? In the Netherlands I've discovered only one firm so far that builds (custom) transformers. Thinks like Orange Sprague caps and clipboards are hard to get and expensive. When I start building (5FA6 Bassmann/JTM 45 ?) again I'll probably buy my parts in the US.

Cheers, Niek

9/11/1997 2:27 PM

You could try Mojo Tone in CA, they may may be a little expensive on the consumer side, I get stuff from them wholesale because I buy lots of Carling switches from them for some overdrive pedals I build for a local store. They seem to keep a good stock of transformers though, maybe you can fix a few amps and call yourself a repair shop for the pricing. Anyway their website is www.mojotone.com and they have a catalog online, the prices are list on the website though. Really nice guys too!
There are some listings of places on the web, go to harmony-central.com, click on guitar, then check out some of the FAQ's under tube amps, it's how I found this site.

Later,
Roger

9/24/1997 8:56 AM

Re: Which preamp to match 2xEL84 class A ?

Sounds like a cool project. As far as the Cathodyene phase splitter, I built a 6v6 amp with that way (I wanted more saturation from the extra tube half). It sounded horrible if the phase splitter was overdriven, And sounded great after switching to a two tube phase splitter type.

There could be a couple of reasons for this though: I may have just needed more headroom and 6v6's require more drive than el84's. I know the peavey classic 20(el84's) uses only one tube for the splitter, and that sounds quite good.

Transformers: I have ordered Hammonds from Antique Electronics- I don't complain about the price. Sorry no address on me, but I have seen it on the web somewhere. peace, eth

9/24/1997 9:44 AM
M Helin	It's maybe the cathode-biasing that makes your amp perform (distort) like it does. Try use fixed-biasing, it should help a lot.

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