Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on August 11, 2023, 02:29:32 pm
-
Got a friend e-mailing me about this amp he's trying to use it for guitar. I'm talking about it in it's original state. He complained of low output and generally bad sound. I told him to clean tube pins/sockets, jacks, try different tubes, different cords and guitars to see if any of that made any difference. This is his response back. I found the schematic and it's really weird IMHO. Seems like it needs to be gutted and rebuilt something else but---I thought I would run it by yall :notworthy: He is very limited in what he can do. So do you see anything he could do simply, to put some life into this amp??? This is what he said:
Took me a while, but finally cleaned the tube sockets and tried a different 6V6 and 12AX7 in the VM amp. But, the guitar still has low output, while touching the tip of the jack on the plugged in cord with my finger is loud as it ever was. Also, I just noticed adjusting the treble and bass controls aren't doing anything, Doing a full sweep on both and no difference in tone. I have already replaced maybe 12 years ago the cap can and every cap and resistor in the thing except the ceramic disc caps on the tone controls and the resistors on the funky input jack. Tried a different guitar, cord, and speaker too. This as far as I know how to go, except maybe just replacing those tone caps and the input jack with a Fender conventional input setup? Still don't know how to check voltages, etc
Attached is the schematic
-
Well, haven't got any comments on this. Not sure what that means. I take it everybody agrees it not a guitar friendly circuit!
But I have some questions myself about a strange circuit?
First---the tone stack. Never recall seeing it tied into the V1a Cathode like this. I understand about summing the treble or bass frequency to ground depending on adjustment which appears what they are doing here but---it's usually through the grid and not the cathode. Also it appears to me that if you plug into the mike input, you bypass the tone stack and the tone stack is only engaged by plugging into the Phono input??? One more---wondering about the tone stack tied into the yellow OT speaker output---NFB?
Second----Power tube Cathode resistor. Never seen the cathode tied into the output transformer before to get resistance(32 Ohm) for cathode. It's low resistance for a cathode resistor. My champ it 470
So that's about it. Maybe someone can correct me or verify what I'm seeing :dontknow: Thanks!
-
Plate - take a look at this thread. PRR is on there.
https://www.diyaudio.com/community/threads/voice-of-music-conversion.320360/ (https://www.diyaudio.com/community/threads/voice-of-music-conversion.320360/)
It seems there is the makings of a Champ type amp there - replace the weirdness with your own, or with something standard. Ground the 6V6 through a 5W resistor? Add NFB as needed?
-
Thanks but you misunderstood. You need to read original post. I wasn't wanting to change this amp myself. Just trying to understand how the existing works myself. My friend is in Cleveland Illinois and very limited on what he can do to an amp. I've built a couple of amps for him in the past. So I was trying to see if there are some simple changes he could make, that would perk up the amp. He explains how it's behaving on the original post. My first answer to him was, the amp needs to be completely re-worked but I thought somebody could see some simple fixes that I didn't see, I could pass on to him before I say "Sell it and get what you can out of it"
BTW--I had seen that post you linked by PRR on this amp when doing research on this amp.
Yeah the guy said when he rebuilt the amp F51 champ, it sounded fine.
-
I think your friend is right to try changing the input impedance to something more guitar friendly.
The tone seems to work off the negative feedback loop.
-
I don't think there is any simple change to make that amp guitar friendly. It has all the stuff to rework and make a Champ. Sounds like your friend is not up to the task though.
-
OK, Thanks. That verifies what I originally thought :thumbsup:
-
OK - I looked at your first post again. Your friend said that he had replaced every resistor and cap in the amp. Thus I assumed he had enough knowledge to make it into a champ type circuit. But, maybe not. :dontknow:
-
Yeah, have tried to encourage him in a complete conversion or build and I think he going to go for it but backs out. Thinking about the whole conversion can seem overwhelming but if you just take it a step at a time very patently like on a champ and not afraid to ask a lot of questions like-- input jack to first gain stage--step one, tone stack to second gain stage step two, any rework of power amp(6V6) step 3 and step 4 any adjustment required to power supply to the tubes & OT.
When casey4's (if any of you remember him) was trying to talk me into a champ project I was very resistant because it seemed like a mountain to me and I had no electronic background. When he offered to hand make me a kit complete with instructions,I took the plunge. So he ended up helping me step by step on the cab and tolex to. So once I did that I was completely sold, been tube ampping every since. I even drew this drawing of all the hookup because I wanted to see where everything was going and what voltage was going where. It blew my mind because I couldn't imagine AC and DC in the same wire, so when I learned that the coupling cap blocked DC and let AC pass, was a big revelation for me :laugh:
-
This should work. Keeps all the tone controls as-is - moves the volume control, adds a 33K and a 10uF cap.
Pay the freight both ways and throw in a 1/5th of Jameson and I'll make the mods. :icon_biggrin: Specs delivers with Door-Dash. :d3:
--Pete
-
Thanks Pete! Appreciate the schematic too! :thumbsup: I believe this might be simple enough where I can walk him through it. I don't know how much difference it will make but its got to be a lot better than the present setup and definitely more guitar friendly. I'll pass it along to him and see if he wants to go for it :happy1:
-
Thanks Pete! Appreciate the schematic too! :thumbsup: I believe this might be simple enough where I can walk him through it. I don't know how much difference it will make but its got to be a lot better than the present setup and definitely more guitar friendly. I'll pass it along to him and see if he wants to go for it :happy1:
Almost forgot: He REALLY NEEDS to add a 3/4-1A SB fuse for the mains... And of course, the three wire power cord.
--Pete
-
Just considering accomplishing the vol control re-location. The physical construction of this amp makes it difficult. Attached is the front view and side view of the amp to give you an idea of this. As is the volume control(top knob) and 1/4 mike input jack(top right) is within inches. When you move the volume between the two gain stages it requires the following:
1-A grid run from input jack down to the bottom of the amp to a 33K resistor to 12AX7 pin 2
2-Disconnect one leg of .047 V1a coupling cap to V1b, remove 2.2 Meg resistor to ground, create a terminal to connect .047 leg to grid wire at bottom of amp routed back to top of amp to input term of vol pot. Then make another grid run from vol pot output term to bottom of amp to 12AX7 V1b term 7
3-Plus with these long grid runs it probably needs to be shielded cable with one end of each run of shielded tied to ground.
4-So this would be three long runs of grid wire. One for input grid, one for send back to volume input and one more back from vol to V1b.
Because of the configuration of the amp this makes mod more on the difficult side! Any brain storm
solutions? or is it "Tough Cookie Jack!"
-
Thanks Pete, yes if he's going to be using it, it definitely needs a fuse holder, proper fuse and three conductor cord. I didn't even notice it didn't have a fuse :dontknow:
If he don't want to go for it, I might make him an offer on it. I been eyeballing those little
compact amps for ages
-
> three long runs of grid wire.
Do you need to run it flat and wide open?
Stand it up. Fill the open part and put the knobs there. Right in front of the small tube. Wires are short, and can probably be routed away from buzzy power.
(https://el34world.com/Forum/index.php?action=dlattach;topic=30646.0;attach=108101;image)
-
Yeah that will make the 12AX7 right next door(short run) and the control panel will be vertical like a 1482. Call it the "Plate Fire Extinguisher"or something like that :think1: And they Lived Happily ever After :m8 :m10 :guitar1
-
I have let Bear (his internet name) know what sluckey and Pete said about the possibilities of simple changes to improve his amp for guitar. Haven't heard back from him yet
I just realized we discussed this very same amp for the same person way back in 2016 as per thread:
https://el34world.com/Forum/index.php?topic=20321.0
So if he don't tackle it this time, I might offer to do it myself just to put it to rest