Unlugging V1 makes it virtually all disappear. I have to put my ear up to the speaker to hear it. Adding one valve should not be too much of a problem. there will be 1/2 over, however. Perhaps something can be done with it... In fact, I already know: another gain stage for Normal Clean; it's too low going into the FX loop. A booster in front or a more complicated level control would have to be added. This was done on a Soldano SLO-100 for Eric Clapton.
True. The gain with local feedback is 10 at best. No good for V1 and V2.
Decreasing those does reduce gain. Increasing them increases gain at lower frequency points, but still allows all the highs in, if I understand correctly:
For the EF86 Pentode idea: The Valve Wizard -Small Signal Pentode
Thank you for all these ideas.
I suppose it will only reduce the lower frequencies that does not produce hiss
so, basicly it only means increasing the hiss, in relative terms
btw, pardon if its a silly question, but is not usually the input tube that has no cathode bypass cap, and the output tube that has a bypass cathode cap ?
is that a normal situation
Not for me. I have to use one 20w 5 inch speaker at home. I do need the gain, though. I would like to achieve an almost square wave. It has limited use, but can be "softened up" with reverb or echo.
Example of near square wave (probably electronic overdrive): Bruce Springsteen - Badlands (Live In Phoenix '78) [The Promise] - 1080p HD! - YouTube The lead starts at 2:33.
so you are not using the upper tube, V1A ?
and further, you also have two channel output ?
I don't know much
but I got less noise when I left the 2 channel output, and settled with just one channel
V1A is the Normal channel; Clean or Crunch. It is out of the chain during OD, as LDR5 would be open.
The Normal channel in Crunch is much quieter than OD. I would not consider making only one channel. In fact, I would like to have two FX loops and two tone stacks; a completely separate chain for each channel!
This way, you could put two different boxes (or more) in each loop, dial in all the settings for optimum and just toggle the footswitch. If you want the same effect on both channels, two separate boxes would be needed.
This scheme is more complicated and more expensive, but it also solves many problems.
Hi, now I' no expert on any of this but I've built a few tube guitar amps, most recently a 50 watt, footswitchable Caswell #39/ Levi #36 type thing (as used by Slash on appetite for destruction) which I suppose is kinda hi gain. Now I think ur hiss is coming from the cathode bypass value used for the 1st stage. The value of the cathode bypasd cap is used to tune that stage's frequency responce, & ur chosen cathode values of 1k8/0.1uf gives a roll off point of around 884hz. U said about Marshall using a 0.68uf, but coupled with a 2k7 resistor this gives a roll off point of 86.69hz. If ur after a similar 3db roll off point using a 1k8 resistor, use a 1uf for a roll off at a slightly higher 88.4hz. U wont get the same "raw" kind of sound as a Marshall & it'll probably also feel a bit tighter. The Marshall' raw sound & looser feel come from using the higher 2k7 resistor value. Cheers
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It is 1uf. And whether it's increased to 11uF or removed completely, the hiss is still there. The hiss sounds like white noise. I will try a 5751 in that position and see what happens. Also, I will try some supposedy quieter 12ax7s for V1 and V2.
The 6N2P-EV is heavy duty for military use. None of the ones I have are microphonic, and all have the same performance. Who knows if they are less noisy than other types, unless they are replaced?
Even if it was 0.1uF, the high frequencies would still come through. I provided a chart above.
Thank you for your input.
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Ok, my appilogies, I thought it said .1uf, I'm veiwing it on a phone. Just while I,m here, the bright Marshall channel can sometimes be a little prone to hiss. Resistor types, lead dress & especially the tube can all contribute. When I first fires my last build up (the circuit is know to be prone to squeal), I found it had hiss & squeal at higher gain settings. Luckilly it was only the 1st tube, the troubles went when I replaced it. Hopr u gey it sorted. Cheers
forgot to ask ...
is this with guitar plugged in ?
and does it change if you touch the strings ?
(when not playing my preamp control is always turned fully down)
I suppose it will only reduce the lower frequencies that does not produce hiss
so, basicly it only means increasing the hiss, in relative terms
You don't really want frequency selective feedback in that way, the cathode decoupler should be chosen to give full gain at less than the lowest frequency you're going to be using.
Using too small a capacitor gives bass cut, which is effectively treble boost - making the hiss worse.
Which is what you would expect, hiss comes from the front end, and is amplified as it passes through the rest of the amp.
Replacing the front end with a single EF86 would probably help, noise is why you don't use triodes in the front end of mike or tape preamps. Also use metal film resistors in the front end, as they are quieter.
But for the gains and overdrive you're obviously looking for, hiss is probably fairly normal, and to be expected (and is part of the 'valve sound').
yes
for music or PA .... no
but for instrument amp it can be used that way
Even for instrument use it would be a bad idea - if you truly want to restrict low frequencies?, there's far better and more effective ways (the roll off rate is pretty poor).
its well described on valvewizard.uk, in every detail, calculation and all ...
but why is that so interesting ?
I would rather hear the answer to my previous post
but maybe it doesn't matter
its clear now the preamp is too complicated to be just about cathode bypass caps
but why is that so interesting ?
I would rather hear the answer to my previous post
but maybe it doesn't matter
its clear now the preamp is too complicated to be just about cathode bypass caps
means removing both V1A and V1B ... right ?
have you tried to disconnect only one of them ?
btw ... regarding cathode bypass caps
isn't it normal with no bypass caps at all, in a guitar preamp ?
means removing both V1A and V1B ... right ?
have you tried to disconnect only one of them ?
It's a LOT more work to disconnect just one, than to simply unplug the valve.
The other half is only on the over drive channel anyway.
Some do, some don't - it really depends how much gain you want.
I suspect the main problem for the OP is that he's adding TOO much gain - just checked some Marshall circuits, and there's usually either one bypassed and two not, or two bypassed and one not (the cathode decoupler on the first stage was also 330uF, but the resistor was lower).
Check this out folks!
A bare wire was attached to V1 first stage input grid and held in open air. The hiss went away but picked up 60 cycle hum as it should.
I suppose that answers the question the source of most of the hiss: RFI.
My theory is that the literal antenna somehow cancelled most of the RFI in the circuit.
So it's there even with no cable connected to the input. This leads me to the power cord, or the power grid at my location, which is an apartment building. I will try the recommended 3-wire EMI/RFI filter and a shieled power cord.
Also, decreasing the value of grid leak resistor (R13) on V2.B noticeably reduced the hiss, and also reduced the gain slightly. At least none of the tone was lost. The reduced gain is really not a problem, since Normal Crunch is considerably lower in output compared with OD. So it makes for a simpler SEND level control.
Well, it probably wasn't any of the valves, so soon I will have a 5751 and a couple of Mullard 12AX7 reissues to play with!
A bare wire was attached to V1 first stage input grid and held in open air. The hiss went away but picked up 60 cycle hum as it should.
I suppose that answers the question the source of most of the hiss: RFI.
My theory is that the literal antenna somehow cancelled most of the RFI in the circuit.
So it's there even with no cable connected to the input. This leads me to the power cord, or the power grid at my location, which is an apartment building. I will try the recommended 3-wire EMI/RFI filter and a shieled power cord.
Also, decreasing the value of grid leak resistor (R13) on V2.B noticeably reduced the hiss, and also reduced the gain slightly. At least none of the tone was lost. The reduced gain is really not a problem, since Normal Crunch is considerably lower in output compared with OD. So it makes for a simpler SEND level control.
Well, it probably wasn't any of the valves, so soon I will have a 5751 and a couple of Mullard 12AX7 reissues to play with!
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