I'm not used to this whole sideways drawing of the tubes in the schematic. It makes it harder for me to follow sometimes. I'm a little confused by what removing that 120K resistor is doing. Well, I mean, obviously from what you said, it's increasing the gain, but I guess maybe the way the schematic is laid out and with the reverb bypass right there, I'm having trouble picturing what its direct effect is. Is it changing the screen voltage on the 12BH7 or allowing a stronger signal to bypass the reverb?
Yep, allows a stronger signal to bypass the reverb by increasing the resistance to ground. It has the same effect as increasing the value of the grid leak on a normal gain stage or turning up a volume control. The 12BH7 has no screen, just a dual triode run in parallel. And it doesn't change bias either. This is set by R310 for 10v. Those 10k resistors are grid stoppers for the 12BH7.
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Sorry, I meant grid voltage, not screen voltage. Thanks. Makes sense.
Normally, I can look at a tube amp circuit and I can easily identify the parts and see how everything fits together. Probably because they're mostly very similar. Something about the tubes being turned sideways on this really messes up my perception of the circuit.
Normally, I can look at a tube amp circuit and I can easily identify the parts and see how everything fits together. Probably because they're mostly very similar. Something about the tubes being turned sideways on this really messes up my perception of the circuit.
So it looks like disc caps that need to be replaced are:
C302: .02uF
C304: .0012uF
C305: .02uF
C307: 100pF
C309: .02uF
C310: 220pF
C314: .001uF
Except for a few mylar .02uFs, I don't have the other values in high enough voltage, so I'm going to have to order replacements. Should I get ceramic discs or something else?
C302: .02uF
C304: .0012uF
C305: .02uF
C307: 100pF
C309: .02uF
C310: 220pF
C314: .001uF
Except for a few mylar .02uFs, I don't have the other values in high enough voltage, so I'm going to have to order replacements. Should I get ceramic discs or something else?
And another question. I just realized (after soldering the first one in, the reverb one, so it's only at about 255V) that my 47uF caps are 400V, not 450V like the originals. Should I get 450s instead or will 400 be okay? It still gives me a 60V clearance over what ought to be the top voltage. That said, the transformer was made for what, 110V? vs 120? So that's about a 9% increase. So maybe a 30V clearance. Maybe a bit too close for comfort, at least on that first filter cap?
Oh, and missed:
C315: .0022uF
Where is C315 in the schematic? I see C314 (output transformer) and C316 (PS filter), but I can't find C315.
Also, C314 is labeled 2KV? I need a 2KV cap there?
Oh, and missed:
C315: .0022uF
Where is C315 in the schematic? I see C314 (output transformer) and C316 (PS filter), but I can't find C315.
Also, C314 is labeled 2KV? I need a 2KV cap there?
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I guess on second thought, I kept the disc caps in the pF range because they most come in this style even new. I didn't replace C304, 307,310 and the 2kV C314. I replaced all .02 value caps with 400v Black Beauties from an organ I scrapped out. Lots of resistors, caps and wire from it.
I don't see C315 either.
I think I would spring for the 450v caps for the first two nodes, plate and screen supply. I like a little more safety factor than the 400v would give.
I'm not sure where you get your parts but I use these guys. The disc caps for 2kV are 30 cents apiece. Or 25 cents for 3kV. They probably went with this high a value because of what was available.
https://www.tubesandmore.com/products/capacitors?filters=Type=Ceramic Disc
I don't see C315 either.
I think I would spring for the 450v caps for the first two nodes, plate and screen supply. I like a little more safety factor than the 400v would give.
I'm not sure where you get your parts but I use these guys. The disc caps for 2kV are 30 cents apiece. Or 25 cents for 3kV. They probably went with this high a value because of what was available.
https://www.tubesandmore.com/products/capacitors?filters=Type=Ceramic Disc
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Yeah, I seem to be getting regular shipments from Antique Electronic Supply these days, much to my wife's chagrin... I'm going to leave the big caps on the chassis and just shove the new caps inside the chassis (the one that I put in on the reverb has heat shrink covering the legs to keep it from bumping into stuff and causing problems.
I have some 400V .022uF mylar caps (new from somewhere). Could I replace the .02 ceramics with those?
When the rain stops I'll take it out to the shed and drill some holes in the chassis. I'm going to move the reverb connectors to the side and put some pots in the front for gain and volume and a 1/4" jack.
When I cut the power cord off (replacing with a 3 prong), the insulator on the wires practically disintegrated. Left a pile of flakes and dust... Glad I made that call!
I have some 400V .022uF mylar caps (new from somewhere). Could I replace the .02 ceramics with those?
When the rain stops I'll take it out to the shed and drill some holes in the chassis. I'm going to move the reverb connectors to the side and put some pots in the front for gain and volume and a 1/4" jack.
When I cut the power cord off (replacing with a 3 prong), the insulator on the wires practically disintegrated. Left a pile of flakes and dust... Glad I made that call!
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I'll replace the NFB pot with one I can put a knob on and I'll call it what it is. Instead of removing R309, I'll replace it with a 100K pot and call it gain.
I'm not sure how best to handle a volume pot. Could I stick one between C309 and the second V8 grid? Would that work or maybe before the first V8 grid?
I'm not sure how best to handle a volume pot. Could I stick one between C309 and the second V8 grid? Would that work or maybe before the first V8 grid?
Happy days... Just found a couple 47uF 450V caps that I had bought from China a while back sitting under a stack of papers. How's that for luck. The problem is, one has a small dent in it. I believe it happened in shipping.
Here's a profile shot of the dent. It's pretty minor and there doesn't appear to be any other damage to it. The top isn't bulging or anything. Any thoughts?
I suppose I could fire up that 230V Antek transformer I have, and setup a rectifier circuit and run 300+V through it for a bit and see what happens.
If I put two 1W, 100K ohm resistors in series as the load, that should keep the power down to about 1/2 W, right? Could I safely do this on a breadboard (haven't done much HV stuff yet, other than work on existing circuits). Sorry to ask so many questions, but I'm a big fan of staying alive
Here's a profile shot of the dent. It's pretty minor and there doesn't appear to be any other damage to it. The top isn't bulging or anything. Any thoughts?
I suppose I could fire up that 230V Antek transformer I have, and setup a rectifier circuit and run 300+V through it for a bit and see what happens.
If I put two 1W, 100K ohm resistors in series as the load, that should keep the power down to about 1/2 W, right? Could I safely do this on a breadboard (haven't done much HV stuff yet, other than work on existing circuits). Sorry to ask so many questions, but I'm a big fan of staying alive
I don't think anyone can hear the difference between a .02 and a .022 cap.
As far as that cap goes, I'm not sure of the quality of the Chinese caps. I know Gerald Weber says take out all Chinese filter caps, they are junk. So I'm not really sure about what to think. The dent is pretty small. I suppose it's a bit of a small risk.
Yes, those power cords almost always have turned to small pieces, must have been the plastic they used. I have not kept a single one from Hammond amp.
As far as that cap goes, I'm not sure of the quality of the Chinese caps. I know Gerald Weber says take out all Chinese filter caps, they are junk. So I'm not really sure about what to think. The dent is pretty small. I suppose it's a bit of a small risk.
Yes, those power cords almost always have turned to small pieces, must have been the plastic they used. I have not kept a single one from Hammond amp.
Yeah and Gerald CAN be quite opinionated. But he DOES have a wealth of knowledge too. I've learned a lot of practical stuff from his books. He's been around the block a few times. Lots of good everyday advice. You could always use the cap for cathode bypass for power tubes or any tube for that matter. At least you won't have the high voltage on them.
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