Guitar Tube Amp Conversion Questions

[ToobNube]

I have some stuff on the way, so sometime in the next few weeks I'll find some time and see what i can do. The first thing is remove and replace the little board with resistors on it and give the bigger caps a real home instead of just hanging out. Then mount it and wire it up.

Thanks for all the help so far. I've been learning a lot.

[ToobNube]

While I'm waiting for parts in the mail, I want to brainstorm what the finished amp will be like. I decided to restore this amp back to schematic specs as a Hi-FI amp, but that begs the question: how am I going to use this thing for guitar?

In my head I'm envisioning a "best of both worlds" scenario that would allow me to use the amp combo for both home-audio and as a guitar amp. In this scenario I would be fully restoring the amp back to specs with no deviations (except possibly a NFB cutoff switch). When I want to use it as a home-audio speaker, I simply plug the turntable into the vol/tone "Control Panel", which then is plugged into the amp (which has no vol/tone or any pots at all within the circuit). To switch to use it as a guitar amp, I'm thinking I would unplug the RCA cable (connected to the Control Panel) from the amp, and plug in a standalone preamp, which would serve to prepare the signal for the amp as well as give it some harmonic distortion the very clean hi-fi amp may not be able to offer. I'm also thinking about installing a switch on the amp to turn off NFB when I use it for guitar, but I haven't decided.

Is this a sound plan? Building a standalone tweed-style preamp in a box to use when needed? I suppose I could also use it with other amps in the FX Loop as well. I'm thinking simple: transformer, one 12ax7, volume, tone stack, master volume, preamp on/off, and pilot light. Basically a clone of the 5f1 preamp in a box with added tone stack and without NFB on the cathode of the second gain stage. I briefly considered adding it all to the amp, but with the amp sitting almost on the floor, i'd either have to bend down a lot to adjust anything or have a lot of cumbersome wires in the cabinet (which already exists to a certain degree due to the Control Panel), so I think standalone works better, although the rub is I need to power it separately. I believe I found a suitable transformer in the fender reverb standalone, though (https://www.tedweber.com/wrvbpt/).

From what I can tell the main weakness in this plan is the speaker. I have no idea how this woofer/tweeter combination would sound as a guitar speaker, and I don't know how a guitar speaker would sound for home audio/HiFi. I need to find a speaker than can do both and can switch between the two with minimal user effort. I don't want the user to have to do any more than flipping a switch inside the speaker cab.

The other weakness I see is that I don't know what signal level the amp expects to see at the plug, which feeds directly into pin 1 of the 6au6. The goal with the preamp would be to make it not just for this amp, but also for other amps (using FX Return), so I don't want to tailor the outputs (of the preamp) to this amp so much that it obscures its ability to perform well in other situations. But does that mean I need to make another adapter to go between the preamp and the amp? I don't know yet, but I'm not against it if that's what it takes. For example, there it no grid leak resistor or grid stopper resistor on the amp before the 6au6, and I'm not sure I want to add it into the amp itself due to the HiFi signal hitting the control panel first, so I might need to make an input adapter box for the preamp.

Any thoughts on any of this?

If I could figure out a way to use this amp for both purposes, this would make an amazing addition to my living room.

[35L6]

Search Alembic F 1x .

[The Dude]

As far as having a tweeter for hi-fi use and not for guitar, a simple switch on the tweeter ought to work for that.

On using the turntable direct in: Many turntables have a pre emphasis circuit and corresponding preamps/amps have a de emphasis circuit. The EQ on the front end of your unit may be enough to compensate for this, but you may need to build a de emphasis circuit for a turntable direct input.

For hi-fi use, this is a mono amp, so no stereo. I wouldn't stand for it, but if it doesn't bother you,.......

As far as building a separate guitar preamp, IMO it would be easier to build an input on the existing unit for guitar and use a stomp box, but if you prefer a separate tube guitar pre, I get it.

[ToobNube]

no stereo... it hadn't occurred to me... Yeah, that's not going to work for me either.

Thank you for pointing me towards Pre-Emphasis and De-Emphasis circuits. That's exactly what I was looking for.

Thanks for the tip on f1x. that's basically what i'm looking for, so I might use that as a template or starting point if i go that route.

I'm going to have to think about this some more. I'm still committed to restoring this thing back to schematics, I'm just not sure where to go after that.

[35L6]

If you don't restore it you will always wonder what it sounds like . If it sucks you'll have no qualms about Voxing it .

[ToobNube]

If you don't restore it you will always wonder what it sounds like . If it sucks you'll have no qualms about Voxing it .

I agree. Even though it might not be exactly what im looking for, this project has been very valuable to me so far as a learning experience, amd im going to see it through. It would feel pointless to gut it and wire up a common circuit. If id wanted to do that i would have just bought a kit. I'd rather restore it and sell it and buy a kit than force this amp to be something it isn't and lose everything special and unique about it.

also i haven't mentioned it but i have another tube amp project on deck to work on after this is finished, a 30 watt knight pa tube amp i picked up at a thrift store about 10 years ago with the dream of one day converting into a guitar amp. I'm not going to dig into it until im finished with the VOM project, but i suspect that amp may be better suited than this one for a guitar conversion.

[ToobNube]

i may or may not have set up a time on sunday to go look at two full tube-powered stereo/turntable consoles (1 hifi, 1 not). there's ostensibly plenty of room in the basement, but my plate is pretty full right now with various projects. the price is just too good to pass up.

would love to restore both, keep one for myself and give the other to a good friend whose hobby is collecting and listening to vintage vinyl.

i was supposed to be making a guitar amp. what is happening.

[ToobNube]

ultimately chose not to buy the new projects. too much on my plate right now. parts for VOM should be here by Friday.

[ToobNube]

parts came in. i spent some time putting the new board together, but it's not ready yet. i need to cut it for size and also remove some things inside the chassis.

some of the things i'm seeing with the wiring are clearly different from the schematic, and some of the wires appear to be newer than others. what degree of faith should i put into the schematics vs. the actual wiring i'm seeing? my guess is that it stopped working at some point and someone hacked together a fix instead of replacing whatever component blew out. would it be ok to simply wire this back to schematics? are schematics from this era reliable?

also i have a very specific question. there seems to be two C16s on the schematic: one in parallel with R4 leading to the V1 plate (C16A), and the other before the tweeter (C16B). What should to voltage rating for these caps be? I'm guessing about 300V for the C16A, and 100V for C16B? Does that seem reasonable?

I ordered a few wrong parts, but I have enough to get started. Thanks.

[ToobNube]

Due to limited space I thought this tag board would work well. Before I'm finished I will solder the components and add jumpers as needed, but i just wanted to do a gut check. Does this look normal? Is this a valid method of wiring up an amp?

[loudthud]

There is at least one mistake on the schematic at the Bass control. Maybe others. Be suspicious anywhere you see two wires crossing without the big dot or the jump-over symbol.

[ToobNube]

I see that. I'll definitely look out for that in the future.

I've been thinking that the tagboard i got is just too big for this chassis, so im going to get something more compact, similar to what is in there now. If im going to relocate all of the components neatly onto a board, i need it to be smaller. There are probably about 10 or so resistors and caps just hanging out, and it looks bad and i want to.clean it up. Once i get the new tagboard I'll solder components and start wiring up the amp. My hayseed hamfest multi-can cap should be here soon, so that's cool. Slow process, but im still learning a lot. Im spending a lot of time looking at various guitar amp schematics, and reading circuit analyses. The more I look at this amp's schematics, the more it makes sense. I feel like im learning a new language, which is cool. I can't wait to get this thing working!

[ToobNube]

Hey, question. There's a hum balance pot on the schematics that looks to be wired wrong and also hollow/missing. It calls for 150 ohms, but im having a tough time finding anything other than 100 ohms. That's ok for a substitute, yeah?

[g1]

Does the heater winding have a grounded center-tap as shown in the schematic? If so, I think that pot is specifically for V1 as it is wired up to the suppressor grid. 100R pot should work, but it will be loading down the heater winding a bit more than the 150R would be.