Rebuild - Organ/guitarr-amp what scheme should I try?

[überfuzz]

Hi

I've got a home brewed amp that I'm not really satisfied with. The idea was to build an amp to use instead of a twin reverb while gigging. As it is now I drag around the twin... I play guitar mostly, but now and then I plug in an organ.

Now I'd really like to get some ideas or pointers on what scheme to aim for in the new project. The chassis is kind of gnarly. A small board will fit as the amp is, a bigger would mean some rebuilding, replacing of parts.

Plate in the old: 360VDC

[Chuck H]

What isn't satisfying you about the amp now?
What size is the amp? Watts?
What features do you need? Reverb, channel switching, trem, etc???
How many clean watts do you need in order to leave the Twin at home?
Will you be satisfied with a smaller amps clean stage tone?
What is the preamp tube compliment?

[überfuzz]

What isn't satisfying you about the amp now?

It's quite instable...

What size is the amp? Watts?

It's about 33 Watts, at least that's what the present scheme said, it's a rebuild.

What features do you need? Reverb, channel switching, trem, etc???

Now you're getting down to the hard questions...
As of now it has:
Reveb, I don't know the tank-model but I'll look it up asap.
No trem, but I really like it though...
Two channels, pot places in the front panel.

How many clean watts do you need in order to leave the Twin at home?

Hmm, I'm not very good with numbers... But maybe a channel switch could come in handy. I like it to be on the clean side on both channels, but not sterile. ;-)

Will you be satisfied with a smaller amps clean stage tone?

Depends on occasion really. Big venue Twin, I've felt that I could get away with a smaller machine many times playing smaller places.

What is the preamp tube compliment?

It used to hold 4 12AX7s and 4 EL84s.

The chassis and all the hardware is a Carvin vintage 33 amp. I know I have a link to the scheme but can't find it... I'll post it when I've looked it up. I did a rebuild, on channel from the carvin scheme and one 5F6a, of it a while back but haven't really got it stable. Now I look for something simpler. The present circuit is really bloated...

[imaradiostar]

So you had a Carvin, built a plexi channel and now you don't like it?

Is it otherwise still the Carvin circuit?

jamie

[überfuzz]

No I point to point wired it. I just feel like, why not tinker some more if I don't use it as it is now. I have plenty of stuff (components) to go so why not, right? I just think some supervisory would be good... Like what could be build around the PT I have?

[imaradiostar]

Before we all chime in with advice you may want to give a quick rundown of what guitars, pedals, and keyboard (and effects) are being run into this amp.

Without knowing the details I would lean toward a Marshally guitar channel with a pair of triodes in parallel out front (and switchable cathode caps for different levels of bass rolloff), a gain control, gain stage+cathode follower, standard marshall tone stack, & master volume into a 220k "blend" resistor which connects to the phase inverter input.

On the keyboard side I'd tend toward a triode with a big cathode cap driving a james/bax tone control and a volume directly connected to a second fully bypassed triode. The plate of that triode would drive a .022 to .1 uf cap into a 220k blend resistor also connected to the input of the PI. Depending on the output level of the keyboard I'd want the option to pad down the signal before the first triode and I'd wire the input jacks accordingly.

Also- I'd want something wildly different depending on if we're talking about a mellotron, a rhodes, a moog or a farfisa...different things seem to work better for different amps. For a rhodes I'd NEED trem, no doubt. For a mellotron I'd NEED verb but could do without trem. For a more modern synth with built in effects I wouldn't use a guitar amp at all- I'd build/buy something with a pair of clean power amps and accurate speakers. You can see how one could iterate on this for a while.

jamie

[überfuzz]

Thanks for good input!

Input
Rickenbacker 330 - singel coil, mostly no pedals
Hagström viking - Humbuckers, mostly no effect pedal
Occasionally: Organ, mostly midi+emulator
Seldom: melotron-emulator.

I really appreciate your input. However, I thought of it a bit more like this. Slap my fingers if this is all wrong.

1. Design a power section, if possible with an all-round functionality.
2. Design preams that can drive the power section.

Oh, by the way, I'd like to keep present the reverb tank in the equation. I mean it's sitting there, why not use it.

Cheers!

[imaradiostar]

Do you like a little bit of distortion on your guitar or just clean?

As for the power section- I'd just stick with a vox style long tailed phase inverter. If you want it to be a touch cleaner/tighter you could add negative feedback and modify the phase inverter accordingly.

Does the amp have a reverb transformer? What is the part number of the tank? In other words- if it was originally driven by an op-amp your circuit choice needs to be different than if it were driven by a tube and transformer.

With four tubes you'd have to stick with some pretty simple preamp designs to keep the reverb. You may find that a mosfet or jfet can really help- but then things get decidedly less simple.

What type of construction are you using? Turret board? Eyelet? Vector board?

jamie

[überfuzz]

No there's no reverb transformer. I'll look up the spec.no. later today.

I mostly go with eyelets, but that's no must.

I sure like some distortion, so the part where you vent all VOX + NFB caught my intention. I've seen feedbacks where certain frequencies are damped. Is it unwise to go with a feedback over all frequencies?

I'll look into vox power stages, have to work some as of now.

[imaradiostar]

NFB is overrated, in general, but I thought it might tighten up the power section for keyboard use.

I'd be tempted to build a marshall style preamp with three gain stages for guitar. That way you can get plenty of distortion and roll off your volume for a cleaner sound.

If the reverb originally used op-amps you may want to use a triode to drive it and use an LND150 mosfet to recover it. Check out Merlin (the valve wizard) Blencowe's web site for more info on the SRPP reverb driver.

jamie

[überfuzz]

Ok, time for an update.

The reverb tank is labeled: 9GB2C1B. The chassis is actually quite roomy. About 22cm deep wall to wall and 30cm between PT and OT. (The PT is through chassis style.)

Like you suggested, I'm leaning towards a ac30 styled power section. Not sure of the preamp though. What do you say if I say trem + reverb? Is it possible or just dumb?

By the way I like idea of having NFB. Just don't know what kind to have, all freq or selected. I guess I could find the sweet spots by wiring a big resistance pot and see where it yields. And then wire a resistance + small pot.

Cheers!

[imaradiostar]

That's a high input impedance tank- a good match for a single tube reverb without needing to use a transformer to drive the tank!

I'll have to get back to you when I have more time.

jamie

[überfuzz]

I'll just wait patiently. ;-)

When I get some free time to just goof around I'll scrabble down what I have at the moment. Eh... how the amp looks in my mind, layman mind... Any further input is much appreciated!

[imaradiostar]

So...should I draw up a schematic for you?

I can write something that would theoretically work but you never know till you build it!

jamie

[überfuzz]

If you do that I'm going to nick it Radiostar