noise in guitar amps
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enthalpystudios
Well-known member
Thanks kit, i'll check out that fet.
I would like to restore that amp to its original spec, although I play with another idea from time time. and forgive me if this is totally an idiotic idea (i'm really not that sure about how close the two are), but I think it would be cool to bring the normal channel back to spec, and tweak the 'bass instrument' channel to be something a bit more high gain, plexi-ish without the EL84 (which, I know, is much of the plexi flavor).
Just something higher gain, but using the same o/p tubes.
I've always wondered what, exactly, the channels in the amp 'share.' They can't really be two discrete channels... they obviously sum before the output, probably before the power amp. I guess I just need to find a schematic for it. Hard part, for me, is figuring out what I've got, as the chassis looks like its been reinstalled into the head it has now. So there's some mislabeling going on regarding its circuit. AA165, AB165, not sure.
I'll go check out the datasheet on that 2sk170 though, thanks.
Billy
I would like to restore that amp to its original spec, although I play with another idea from time time. and forgive me if this is totally an idiotic idea (i'm really not that sure about how close the two are), but I think it would be cool to bring the normal channel back to spec, and tweak the 'bass instrument' channel to be something a bit more high gain, plexi-ish without the EL84 (which, I know, is much of the plexi flavor).
Just something higher gain, but using the same o/p tubes.
I've always wondered what, exactly, the channels in the amp 'share.' They can't really be two discrete channels... they obviously sum before the output, probably before the power amp. I guess I just need to find a schematic for it. Hard part, for me, is figuring out what I've got, as the chassis looks like its been reinstalled into the head it has now. So there's some mislabeling going on regarding its circuit. AA165, AB165, not sure.
I'll go check out the datasheet on that 2sk170 though, thanks.
Billy
pstamler
Well-known member
[quote author="enthalpystudios"]I would like to restore that amp to its original spec, although I play with another idea from time time. and forgive me if this is totally an idiotic idea (i'm really not that sure about how close the two are), but I think it would be cool to bring the normal channel back to spec, and tweak the 'bass instrument' channel to be something a bit more high gain, plexi-ish without the EL84 (which, I know, is much of the plexi flavor).
Just something higher gain, but using the same o/p tubes.[/quote]
One way to do that is to install a 1/4" output jack, normalled to pass signal when nothing is inserted in it and to interrupt when something is, after the second tube section, the one which follows the tone stack. Then, when you want, you can plug that output jack into the input jack on the Normal channel and have all the extra gain you like.
Peace,
Paul
Just something higher gain, but using the same o/p tubes.[/quote]
One way to do that is to install a 1/4" output jack, normalled to pass signal when nothing is inserted in it and to interrupt when something is, after the second tube section, the one which follows the tone stack. Then, when you want, you can plug that output jack into the input jack on the Normal channel and have all the extra gain you like.
Peace,
Paul
> saw a real old epiphone in a luthier's window
Nothing to do with today's "Epiphone". Epi long-greek-name was a pioneer in pop music instruments, but he passed on and the company became a brand-name passed around various owners.
> this blackface bassman head I have
If it makes sound at all, it is fixable with little cost. Nothing at all wrong with a healthy Blackface Bassman. It isn't the greatest amp ever made, but what is? Everybody has an opinion, and many people like the Bf Bm.
> what, exactly, the channels in the amp 'share.' They can't really be two discrete channels... they obviously sum before the output, probably before the power amp.
Of course. "Channels" are cheap tricks: it is mostly one amp with two inputs and two sets of knobs.
The general plan of about any gitar amp is: preamp, knobs, driver, output.
Many amps have two preamps and two sets of knobs. These go to a mixer, usually just two resistors. This may go directly to the "power amp", understood to be power tube(s) plus a driver tube to boost roughly 1 Volt signals up to the 10V or 20V signals that power tubes need. Or there may be some common signal processing after the two "channels" are mixed together but before the power amp.
Go to the Stones' concert. There is a 256-input mixer, racks of main EQ and limiting, boatloads of amplifier, and stacks of speakers. Even at this scale, the input "channels" are almost the smallest part of the rig. Cut this down to 50 Watts and 2 inputs, it is still mostly common stages: power amp and speakers.
A "channel" is usually one or two triodes and the knobs. A 7025/12AX7 is two triodes in one bottle, and Fender usually wired the better amps so one bottle was the first channel, the other was the other channel. The third bottle may be mixer/booster, or may be the power amp driver stage.
> AA165, AB165, not sure.
These are 98% the same, but the little differences are significant. Look inside on the tagboard above the 12AT7 driver tube (the last little tube before the big tubes).
AA has an obvious 100 ohm (Brn-Blk-Brn-silver) resistor at right angles to the other resistors. In this area, AB has a 47K (Yel-Vio-Orn-silver) at an odd angle. There are also significant differences in the Bass Inst. channel, the mixer, and the transformer polarity, so a AA-to-AB conversion needs someone who knows what they are doing. I don't know which one sounds better, or which one is more popular.
I do think that if you just fix what's broke, either A_165 is a fine amp, to play or to sell.
Go to www.hoffmanamps.com Hoffman sells amp parts, parts known to work good in guitar amps. Buit first go to his Forum, the first section. One of the top threads is "Post Amp Voltages". Learn how to use a voltmeter, without getting killed, and measure the voltages on the tube pins. Put them in the chart, then start a thread on that board. Someone who knows these beasts will probably help; or I may see something that smells wrong. And describe the problem better than "kind of screwy"... many ways to be screwy! Is it a left-hand coarse-pitch screw, or a fine thread screw, or does it burble when you play low E?
Nothing to do with today's "Epiphone". Epi long-greek-name was a pioneer in pop music instruments, but he passed on and the company became a brand-name passed around various owners.
> this blackface bassman head I have
If it makes sound at all, it is fixable with little cost. Nothing at all wrong with a healthy Blackface Bassman. It isn't the greatest amp ever made, but what is? Everybody has an opinion, and many people like the Bf Bm.
> what, exactly, the channels in the amp 'share.' They can't really be two discrete channels... they obviously sum before the output, probably before the power amp.
Of course. "Channels" are cheap tricks: it is mostly one amp with two inputs and two sets of knobs.
The general plan of about any gitar amp is: preamp, knobs, driver, output.
Many amps have two preamps and two sets of knobs. These go to a mixer, usually just two resistors. This may go directly to the "power amp", understood to be power tube(s) plus a driver tube to boost roughly 1 Volt signals up to the 10V or 20V signals that power tubes need. Or there may be some common signal processing after the two "channels" are mixed together but before the power amp.
Go to the Stones' concert. There is a 256-input mixer, racks of main EQ and limiting, boatloads of amplifier, and stacks of speakers. Even at this scale, the input "channels" are almost the smallest part of the rig. Cut this down to 50 Watts and 2 inputs, it is still mostly common stages: power amp and speakers.
A "channel" is usually one or two triodes and the knobs. A 7025/12AX7 is two triodes in one bottle, and Fender usually wired the better amps so one bottle was the first channel, the other was the other channel. The third bottle may be mixer/booster, or may be the power amp driver stage.
> AA165, AB165, not sure.
These are 98% the same, but the little differences are significant. Look inside on the tagboard above the 12AT7 driver tube (the last little tube before the big tubes).
AA has an obvious 100 ohm (Brn-Blk-Brn-silver) resistor at right angles to the other resistors. In this area, AB has a 47K (Yel-Vio-Orn-silver) at an odd angle. There are also significant differences in the Bass Inst. channel, the mixer, and the transformer polarity, so a AA-to-AB conversion needs someone who knows what they are doing. I don't know which one sounds better, or which one is more popular.
I do think that if you just fix what's broke, either A_165 is a fine amp, to play or to sell.
Go to www.hoffmanamps.com Hoffman sells amp parts, parts known to work good in guitar amps. Buit first go to his Forum, the first section. One of the top threads is "Post Amp Voltages". Learn how to use a voltmeter, without getting killed, and measure the voltages on the tube pins. Put them in the chart, then start a thread on that board. Someone who knows these beasts will probably help; or I may see something that smells wrong. And describe the problem better than "kind of screwy"... many ways to be screwy! Is it a left-hand coarse-pitch screw, or a fine thread screw, or does it burble when you play low E?
enthalpystudios
Well-known member
Very cool PRR, clarified some confusion I've had..... about to check out that forum as well. Acting sketchy in that the 'normal channel' passes no audio, and the bass instrument pots are very scratchy and intermittent. Aside from that, I have no real answers. It, at least, has new tubes.
This is cool, however, I'm going to get on this project on this nice sunday afternoon ;]
Thanks again!
Billy
This is cool, however, I'm going to get on this project on this nice sunday afternoon ;]
Thanks again!
Billy
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