Guitar Tube Amp Conversion Questions

[g1]

It looks like capacitor C5 has 3 prongs. I tested the other capacitors and I'm going to replace them, but im not sure what to look for to replace C5. It has 3 shapes on it with different values for each shape: semi-circle - 40mfd 250vdc
triangle - 10mfd 25vdc
square - 40mfd 450vdc

Look near the lugs and you should see those shapes, one by each lug. That is 3 caps inside a single can. Ground is common and probably the case/shell. You can replace with 3 separate caps. The low voltage section is usually the cathode bypass for the power tube.

[ToobNube]

I removed and inspected all of the tubes. They all look ok except one of the el84s, which has a tiny little black burn out on the inside of the tube near a metal part. The tube might be fine, I dont know. I turned it on with only the 5y3 and it seemed ok, so I put the tubes back in and turned it on hooked up to a speaker. I soldered the end of the audio-in wire to a guitar jack in a pedal enclosure as a temporary jack, plugged in my guitar and turned it on. It takes a few seconds to warm up, but im not getting any signal and theres a wicked hum. I unsoldered the jack and used some wires to solder it directly to the rca input of the amp since i don't have an rca adapter. I wanted to see if maybe the signal was being cut at a bad pot or resistor or something in the control panel. When i turned it on I never really got any signal, except turning my guitar's volume knob made this hideous scratchy pot noise that was pretty loud. The hum was louder but got much softer when i grounded the chassis or guitar by touching it.

[ToobNube]

The triangle cap is reading null, so it'll get replaced. The other two looked ok. All of the resistors are reading ok. C6 on the schematic says "500 600VDC", but i dony see anything like that in the amp. Could it have been removed? It looks like it's in the feedback loop? Would a 500uf 500v work instead? Thats a big boy to just be missing...

[eschertron]

I don't see C6 on the schem. I do see C16 twice.... what's the value of the cap on the tweeter crossover?

[ToobNube]

85 mfd 100vdc on the tweeter

[eschertron]

85 mfd 100vdc on the tweeter

the BOM calls out C16 as 2mfd 100v. 85mfd would turn the tweeter into a woofer, and maybe toast, too! This one sure is a wonder... I hope you like puzzles

[Helmholtz]

The tweeter series cap is most probably not an electrolytic. It needs to be a non-polar cap.

[g1]

The way the schematic values are called out at the bottom of schematic, I think if they do not say MFD, then they must be pico farads?
Like Helmholtz said, there are 2 C16's shown on schem., so a bit of error that confuses things. Maybe they are the same value then I guess it's not an error. One is in series with the tweeter, one is off V1 plate, in parallel with R4.

C6 is shown along the top, in the feedback loop from the OT secondary.

[ToobNube]

If I bring this amp back to schematics, I'm still going to have to create an input for guitar, and the volume/tone controls will still be in the wrong place. I'm thinking I have a couple options:
1. I could re-tool/re-wire the amp into essentially an ac-15. Change the biases, re-work the NFB, adjust the voltages, and add volume/tone in the right places. This would use a lot of the elements of the amp, but I'd be replacing a lot of parts to mimic another amp circuit entirely. The end result wouldn't resemble the original amp much, but this is probably the fastest and easiest path to a working tube amp for guitar
2. I could replace all the caps and bring this amp back to schematic specs. At that point, I'd need to evaluate what the amp sounds like and see what it needs to work as a guitar amp. I'd probably need to create an input stage and possibly even add a gain stage or preamp before feeding into the 6au6, which as a previous poster mentioned, appears to be in the power stage. Adding a 12ax7 and 2 preamp gain stages, for example, would pretty dramatically change how this amp sounds, but I'd at least have the option to keep the core circuit the same. I suppose I could add a input switch to select between using the amp as a Hi-Fi amp with a restored Contol Panel, or switch to use the mini guitar input stage and preamp. This seems like the "best of both worlds" solution to me, but also would be a lot more work, and I'd probably never use the speaker as a Hi-Fi extension cab in the future. If I do this, could I throw the pre-amp at the top of the cab, and have the core amp at the bottom, and power both from the power transformer?

Any thoughts?

[35L6]

I would preserve my options to start with , in other words do the things you need to do for either #1 or #2 , like 3 wire power cord and replacing those 60 year old caps . The more I look at the schematic the weirder this thing gets . I saw the 25 volt dc thing and thought it was a typo , but given the dc coupling it is probably right . Did you notice the suppressor grid circuit ? It is elevated from ground by 150 ohms and tied to a humdinger in the heater circuit . AC of course . I think it may introduce some anti-phase hum to null out hum in the rest of the circuit . I wonder how much gain V1 has . It may simply be a conveniant place to connect the NFB , sets the bias for the PI , and implements the humdinger . Do you have a signal generator ?

[Helmholtz]

The 25V plate voltage looks about right. It sets the grid voltage of V2 which must be a little below the cathode voltage (28V).
The Gm of a pentode mainly depends on screen voltage and only little on plate voltage.

[ToobNube]

Well, defining my options and mulling it over helped me make my decision. Call me a romantic, but I don't like the idea of wrecking this amp without a good reason or plan, so I'm going to start by restoring the amp back to schematics. I have a custom multi capacitor for C5 on the way from hayseed hamfest, and i'm going to replace the rest of the caps and throw everything onto a new strip terminal. I guess I should probably replace the resistors too, yeah? I have some automotive hookup wire in various gauges I was thinking about using, but I wanted to see if you guys have any recommendations since I believe I'll be replacing a lot of the wiring. Aside from getting this to work, I want to make the inside of the chassis look nice and neat. I don't have a ton of space in the chassis, so getting that multi-cap was really important. Any recommendations before I pull the trigger on this order?

[DrGonz78]

I have some automotive hookup wire in various gauges I was thinking about using, but I wanted to see if you guys have any recommendations since I believe I'll be replacing a lot of the wiring.

What is the voltage rating on that automotive hookup wire? 12v-120v at best? Fact is that your voltages in the amp are going to be between 250-350vdc. So I recommend only replacing the hookup wire with something that supports that rating. For high DC voltage in amplifiers I typically use 22awg "600vdc" and even 20awg for a bit thicker wire, 600vdc insulation ratings. When working with AC voltage 120v on amps I use 16awg or 18awg with 300v insulation rating.

https://www.amplifiedparts.com/produ...d-600v-50-feet

[ToobNube]

I'm not sure, I'll have to check after work. on A.P. I've noticed cloth-sheathed wire. Is there any advantage with that over normal wire? or is it just a preference?

[DrGonz78]

The new cloth wire sold these days is typically the same wire insulation with cloth on the outside for aesthetic appeal. Perhaps that brand is made vintage style with a full wax insulation. People that go with cloth style are typically just trying to make it look vintage.