Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RTL on December 15, 2025, 11:36:49 pm
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Hello,
New to the forum and first tube amp upgrade. I bought a 1994 Pro Junior off of marketplace for $160 that is pretty torn up but functional and sounds great. The stock speaker cone had water damage, as does the box, so I put a greenback in it. The v1b tube was microphonic so I put some JJ ECC83's in. I'm going to replace these with higher gain 12ax7's to hopefully get it to break up a little earlier. I put Tung-sol EL84's in replacing the Mesa branded tubes it had. They are biased hot which I read is typical of this amp. I've been playing it with a Carbon Copy delay and a GTO OD pedal in front of it and it sounds great but needs a lot of things. I was going to re-cap it, but since I have to pull the board out to do the caps, input jack and bias I decided to just build a new board because I want to. I'll post progress and hope I can get experienced feedback so I can avoid undue pain and suffering. Kit arrives Saturday.
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Kit arrived today. The attention to detail is evident down to how the order was packed. I got the mechanical details sorted, turrets laced and jumpered and checked everything with the meter. Ordered a new power switch to avoid having to change it any time soon. Broke the led light trying to remove it. Haven’t decided what I’m going to do about that yet. Chicken head knobs don’t fit the new pots so I’ll have to get new ones. So far this is a good first project and cheaper than building a 5E3 kit.
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I worked on this from 9am until midnight last night. I lost a R19 100k resistor and don’t have a spare. I made some mistakes but was able to correct each one that I recognized. My order of operation will be better on the next one. I thought I would have more room to solder with the pot terminals facing the board but that turned not to be the case. I’ll upload a pic of that when I get home for input. I don’t know how the boutique point to point amp builders make any money doing this. I have a new respect. I also underestimated the amount of work on this project. Does the shouldered insulating washer go under the input jack or speaker jack? I am guessing the input jack but could not find anything on it. One shouldered insulating washer and 2 100 ohm 1/2w resistors are the only “spare” parts I have. Any ideas on the 2 resistors? R24 and R25 are 3 watt and are installed.
Things left to do before power that I know about:
- 100k resistor
- build a dim bulb
- trace entire circuit with meter
- FOD inspection
After:
- power up in stages with dim bulb
- fix any burned components
- check voltages and bias
- annoy wife overdriving the amp
- pilot light solution
- long term I might build a cabinet
Open to any critique.
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- pilot light solution
Hoffman sells a LED and bezel that fit in a 5/16" diameter hole. Would look nice on that chassis.
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Some pictures.
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Fixed more mistakes last night. I wired up the heater ground correctly and added heater power to the right spot on v1a…
Built and tested the dim bulb set up. My resistor kit showed up this morning so it looks like I’ll be firing it up tonight.
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Here’s a sound clip of the stock amp. Volume 7, tone 8, greenback 10”, no pedals. American performer tele. iPhone mic, sorry.The amp really comes alive with some delay and an OD.
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Well, it works. Went through the circuit with the meter and found more mistakes. Turns out pin 6 looks like pin 9 at midnight sometimes. Long story short, When I powered up on the dim bulb everything looked good. I started checking voltages and trying to adjust bias and everything was completely wonky. 1/2 value voltages all over the board, the bias pot maxed out at 30mv and I started thinking about changing resistors. I went through all the voltages again, ran all the grounds with the meter and did find a missing jumper on pin 3 & 8 of V2. Powered up and same story. Then a dim bulb lit up in my brain and I realized I was trying to bias the amp with the other dim bulb loading the amp. I adjusted the bias pot back down to 1/2 value and tried again. Amazing. The amp sounds great and a little sweeter to my ear than the original. The tone control is definitely better. I've exhausted my wife's patience for the night so I'll have to wring it out tommorow. There is some hum and hiss but not any more than it had in stock form. I'll start looking into that as we go. I re-routed the heater wires and nothing changed. I'm going to move the main ground point away from the transformer next.
The bias numbers I settled on were 34.5ma at -12.8v and 302.2v, sound right? Tung-sol data sheet says 300v max on the plate but none of the other numbers are lining up with the chart.
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Here are my numbers. Please take a look and weigh in if you see anything amiss, please and thank you. I'm very pleased with the result. The amp sounds better all the way around. I have not played my LP standard through it yet but the tele sounds fantastic. I'll get a sound clip here at some point. Excuse my playing skills ahead of time.