Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Clear Guy on July 20, 2016, 07:49:11 pm
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Firing up my Hoffman PR build for the first time without the tubes and got the voltages below. So far I have taken the HV readings, Set the bias pot to the highest reading (-40.0ma) and put all the tubes back in. When I power it up with all the tubes in I get a tiny audible squeal which seems like its coming from the preamp tubes. I have triple checked all the work so things should be pretty good there. May be a bad tube? Any help would be appreciated>
V1
pin 1 462 v
pin 6 462 v
V2
pin 1 480 v
pin 6 480 v
V3
pin 1 467 v
pin 6 432 v
V4
pin 1 470 v
pin 6 480 v
V5 (6L6)
pin 3 480 v
pin 4 480 v
V6 (6L6)
pin 3 480 v
pin 4 480 v
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disconnect the NFB loop and see if the noise goes away. If it does, swap the OT primary leads.
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No, leave the FB loop alone for now.
We need all the tube pin voltages, not just the plates.
Your B+ string is not dropping dcv as it goes towards the 1st tube, so tubes are not drawing any B+ current.
Could be heaters, tubes can't draw any with no heater voltage.
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I have the NFB on a toggle switch so I will try that and report back. Thx
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Readings from All 9 pins on V1-4???
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Readings from All 9 pins on V1-4???
Yes and all 8 pins on the power tubes.
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If I read his post correctly the voltages he listed are without the tubes installed.
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D'oh. :BangHead: Yes your right Labb, thanks for catching that.
Clear Guy, posting plate dcv's with no tubes installed doesn't tell us much. Please put all the tubes in and post all tube pin voltages.
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Cool I will try it in the morning!
Ok to just let the tube(s) squeal??
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Do you have a speaker connected? Is the squeal actually coming from the chassis or really coming from the speaker?
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no speaker connected. if i take the tubes out the tiny high pitched squeal is gone.
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Don't turn on a tube amp without a speaker or dummy load resistor connected. It could cause considerable damage.
With respect, Tubenit
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no speaker connected. if i take the tubes out the tiny high pitched squeal is gone.
The faint squeal you hear is an oscillation that is probably caused by wrong NFB phase. It's most likely coming from the OT laminations. If you let it continue for even a minute, you will likely damage your OT and/or output tubes.
ALWAYS CONNECT A SPEAKER WHEN FIRING UP AN AMP FOR THE FIRST TIME. Don't use a dummy load because you wont be able to hear what's going on. With a speaker connected, this squeal will be very loud and alert you that something is very wrong. The first time you hear that squeal will be very unnerving. Turn the amp off and disconnect one end of the NFB wire that goes from the speaker jack to the board as lab suggested earlier. If the squeal stops, then the cure is to swap the OT primary plate leads at pin 3 of the output tubes.
Getting the phase right/wrong on a new build is a 50/50 deal. It's not something you did wrong. There's no way to know if it's correct until you fire it up. This is the main reason for having a speaker connected.
Worth repeating...
***ALWAYS CONNECT A SPEAKER WHEN FIRING UP AN AMP FOR THE FIRST TIME.***
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Got it, speaker always connected! The second I heard the squeal I pulled the plug so it was never for more than a few seconds.
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Progress: I did what Stuckey said and hooked up the speaker, fired it up with the NFB switched to off and the sound is gone. The amp is now playable but I am getting a pretty bad hum with the Reverb on through the foot-switch. When I turn off the the reverb through the foot-swith it is silent but turning it on brings a pretty bad hum through the speaker even with the volume off. Probably something very simple. Grounding?
I will still post all my voltage readings from the tube sockets when I get a chance.
Thx again for everyone's help (Time + Energy)
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If you unplug the footswitch does the reverb hum go away? You must use shielded cable for the reverb switch. The trem does not have to be shielded.
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I will check it when I go home and report back.
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Random weirdness: When the foot-switch is connected, I get a either 100% reverb on or total off. The reverb control does nothing and the Trem is barely audible even with the speed and intensity on 10. I am also getting loud audible pops when I hit the foot-switch on/off switches. If I disconnect the foot-switch while in play mode I get a loud pop followed ticking sound from the Trem but no hum. If I power cycle with the foot-switch still out, the ticking is gone and only the term in on. Some is is definitely funky. Not being able to control the reverb is odd. Where to start??
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Something is not wired correctly. Click on the link in my signature line for some good info to verify wiring.
Is this built in a Princeton Reverb Reissue chassis? If so, it would be very easy to mess up the wiring for the reverb tank jacks and the footswitch jack.
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Yes it is a PR Reissue chassis with the 1/4" foot switch jack. This is the wiring I used.
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Also, I did print the layout in gray scale and verify all my wiring was correct using a high lighter before I ever powered it up. Not to say I still didn't miss something.
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Leave the footswitch disconnected and concentrate on the amp first. The FS is not needed for reverb or trem to work. Trem may be weak because of the bias adjustment. Definitely sounds like something is not wired correctly in the reverb circuit. You may have to go over the wiring some more. Don't forget to verify component values too. And measure resistance from grid pins and cathode pins to chassis to see if the measured values agree with the schematic values.
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Yeah I need to get the amp biased correctly cause its still at the highest negative setting. I will forgo the FS for now until I can get everything else going correctly. I will spent a bit of time to verify all your suggestions.Thanks again for all your help, Sluckey. Back in a bit
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Reverb tank hook up to chassis (RCA):
Tank input to chassis input - Tank output to chassis output
-or- the other way around???
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The tank input connects to the jack that is wired to the reverb transformer. The tank output connects to the other jack which is also connected to the footswitch jack.
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Found the problem: I had never put the ground wire onto the reverb pot. :BangHead:
Everything is working now, even the foot-switch. Reverb is luscious sounding. The tremolo is super weak and is almost completely inaudible. I did bias the amp yesterday with the plate voltage at 427 and (V5) 19.0mv (V6) 27.0mv (11.5 watts) so my power tubes are pretty far off from being matched.
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Maybe there's a wiring error in the trem circuit also. The PR trem should be pretty strong. There are ways to make it much stronger, but see if you can find the problem first.
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Found the next problem: 25KL intensity pot installed instead of a 250KL pot. Amazing how many times you can look over something and still miss it!!!
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That's twice today that a 25K pot was used instead of a 250K pot. Low gain in one case, low trem effect in your case. You guys working together? :icon_biggrin:
So, is it working now?
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No association with the other guy. That is ironic though.
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Sluckey, which 1meg resistor is it that I can change out to a 470k in order to deepen the intensity of the Tremolo (hoffman aa1164)?
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R63
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This one?
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yes
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Any reason I get no sound on the vol dial from 0 to about 2/3 and then comes on pretty quickly? I am using the CTS (brass shaft) 1ma pots. I just swapped out the pot for another CTS 1ma and got no change. The reverb is the same way, no verb till 2/3 on the dial. This may not be anything at all, I just thought I would ask. In other words it may to totally normal or some else is funky. Anyone else run into this issue?
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Could be normal. I don't use CTS pots so I have no experience with how they respond. Most people have issues with reverb being too strong on one on the dial. That reverb pot should be 100K linear. A popular mod is to use a 100KA pot to bring the reverb up more gradual. This doesn't seem to be your problem though.
Maybe some of the more experienced guys will chime in.
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What pots do you use?
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Alpha. I have used that CTS 3MRA a few times though.
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The Alphas seem to be almost a better pot in some ways. Maybe I should try one in that position just for the hell of it.
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The Alphas have a very smooth feel. Quality seems to be fine for a guitar amp.
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Any reason I get no sound on the vol dial from 0 to about 2/3 and then comes on pretty quickly? ...
Others said it less direct: this is almost certainly just how the pot is, and is unlikely to change just from swapping in another pot.
It seems many pots have a resistance track which does not materially change its resistance until a few-degrees turn from either end (bottom or top). You can see this yourself in the graphs on page 2 of this Bourns datasheet (http://www.bourns.com/data/global/pdfs/PDA24.pdf).
It doesn't necessarily mean the pot is low-quality, though I do wish they'd all have the resistance taper stop right at either end-stop. More likely it's a byproduct of mechanical tolerances of the shaft/wiper assembly and the pot body (which typically define where the wiper rotation stops at either end) to enable easier mass-production.