Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on March 17, 2020, 02:05:33 pm
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At first fire up, voltage were there except the first stage plates were very high (short?). Went over the amp, found a few minor R value mistakes but nothing to give me any clues for no sound.
Second try, amp played, didn't do much but change the feedback R, (was 20K not 47K) and up the first rail R to 22K from a low 18K, no issues but I noticed it only played on the bright channel side, tried another 12AX7, both channels played. The bright channel was as loud as usual (I have three AO43's I've finished) but he Normal channel seemed like the volume was not quite as loud as the others I build and certainly not as loud as the bright.
I guess I'm asking is it normal in the 6V6 Plexi (Hoffman schematic) for the bass or normal channel to be a little low on volume then the bright? I used .0047uf, 2K7 bright and .1uf, 820 on normal.
Here are the voltages:
Node A 420v, B 418v, C 282v, D 227v. Plates on V1 are normal 134v, bright 175v, K normal .75, K bright 1.38
The resistors to the volume pots measure 270K each "in the amp" but I'm sure I used 470K. Normal? Pain to check this out, could this be an issue.
Plates on V4 415v, screens 418v I have two ground wires under the bd from the bias caps, one goes to the bias point, 10K and
Plates on V5 416v, screens 418V back to the other cap. Usual under the board for the other set of plates.
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right side pic
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(short?).
open, shorts draw lots of I, V drops like a rock
all I got for your dilemma, I didn't like either channel, but when I put them together and forced them to work together, I was reeeel happy
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I'm probably out to lunch again, ha. :laugh:
I used the Hammond old sockets, didn't spray them, were dirty, bad connections. The 12Ax7 that didn't play now plays fine, wasn't bad. As far as the voltage they all look good, the 470Ks to vol pots, a divider, normal to read low in the amp.
I'm really out to lunch on the two channel's volumes, they are biased so different and the biggest difference is I used the Hammond AO43 OT in one amp and an Allen 30 watt upgraded OT in the other. The 30 watt is cleaner with more headroom and sounded better than the Hammond OT which is pretty good sounding. All three Plexi's have the same volumes but the 30 watt OT is a little louder.