Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on July 06, 2025, 12:23:50 pm
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Hey guys!!
I have built this amp back in 2017, and although it works, it does go on overdrive berserk too easily too soon.
So after sometime searching and thinking and overthinking about it, here what i came up, to move the tone stack between the V2 triodes,
I got a few questions:
- The new position of the tone stack is properly set, and if R15 value on V2b is ok?!
- Bias adjust/balance, will those resistor and pot values work on this amp?(circuit from Rob Robinette)
Thank you in advance!
PS. the thread went to a different path.
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Before completely changing the topology of a proven design to something untested I'd encourage you to try something like a 220k resistor on the input lugs of the volume pots and/or a 2.2M-4.7M resistor to ground at the grid to V2A. Even a pre phase inverter master volume after the treble pot can drastically reduce the breakup.
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So methinks getting a plexi sound with the same number of tubes is a matter of having a direct-coupled pair (inverting gain stage and DC-coupled CF) driving the tone stack (and omitting the 'tone stack recovery stage between the TS and the LTP), which is essentially a JTM45 approach. And then if you still want more fizz, taking the V1 tube and cascading that instead.
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E.g. as in Ceriatone Yeti. (Son of yeti schematic - i.e. with 6V6s - shown)
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Thanks for the reply guys!
@stratomaster I will do the suggested modifications and report back
@tubeswell i dont mind modifying the amp to something else as long the amp get clean tone fender like or similar,
not so fan of marshall( i tried not my thing)
That idea to move the tone stack i got from the marshal major, and mostly from fender amps,
I want the amp to have a clean sound at high volume, lots of headroom
also i found this local feedback loop from this video (https://www.youtube.com/watch?v=0wCFmeIaf60)(see picture), what do you guys think?
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dont mind modifying the amp to something else as long the amp get clean tone fender like or similar,
Remove the tone stack recovery triode and use that for a cathode follower connected to the triode before the tonestack. That will decrease the preamp gain and sound more like a 1959 Fender tweed Bassman.
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:w2:
>which one is the tone stack recovery triode ?
Is it in the schem in the first post? that is a modified schematic, not the original from here (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf) which is the way the amp is.
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:w2:
>which one is the tone stack recovery triode ?
Is it in the schem in the first post? that is a modified schematic, not the original from here (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf) which is the way the amp is.
triode after the tone stack (but before the LTP) - i.e., it's 'recovering' gain that has been lost in the TS. In this case it's not needed - especially if you want a cleaner signal.
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triode after the tone stack (but before the LTP)
That is only in the modified schem, the amp still as it is in the original schem, i did not move anything around yet.
But would moving the tone stack as in the modified schem would reduce gain? Also crossed my mind making it a twin like preamp.
I did not yet had time yet to do @stratomaster suggestions
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triode after the tone stack (but before the LTP)
That is only in the modified schem, the amp still as it is in the original schem, i did not move anything around yet.
But would moving the tone stack as in the modified schem would reduce gain? Also crossed my mind making it a twin like preamp.
I did not yet had time yet to do @stratomaster suggestions
Can you link to the schematic you're woking from (to save everyone else having to spend time hunting for it)?
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Can you link to the schematic you're woking from ?
I did :smiley:
the original from here (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf) which is the way the amp is.
the schem in the 1st post (https://el34world.com/Forum/index.php?topic=32857.msg364841#msg364841) is just an idea, if it is possible to do it
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the original from here (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf) which is the way the amp is.
So that's what I'm talking about w.r.t. something more like a Marshall JTM45/Fender 5F6A. It should give you a cleanish sound until you dime the volume past 6 or 7. If it doesn't, then something is wrong. Or if it is 'clean' and you want it 'cleaner' - try swapping a 12AT7 into the LTP slot
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It should give you a cleanish sound until you dime the volume past 6 or 7.
That would be great atm amp goes on berserk OD at 3 or so
If it doesn't, then something is wrong.
We did a trouble shooting while back (here (https://el34world.com/Forum/index.php?topic=24572.50)) and no clue about what it could be,
methinks its too much gain all over the circuit, specially V2 circuit(cathode follower), if it go good , that is my though on modded schem!
Or if it is 'clean' and you want it 'cleaner' - try swapping a 12AT7 into the LTP slot
I tried , no effect.
The amp is far way from clean...
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Is each filter cap node decoupling the power supply properly? (Separate HT supply nodes should be at different voltages)
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I got some numbers :icon_biggrin::
bias -55v
Nodes A 478v , B 478v , C 333v , D 285v , E 267v
I tried @stratomaster suggestion, the resistor from V2a grid to ground, 2.2M and 3.9M the ones i have. can notice a little difference with 3.9 but far from clean.
Here the question again, moving the tone stack to between V2 triodes and eliminating the cathode follower, would it reduce the harsh gain?
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I got some numbers :icon_biggrin: :bias -55vNodes A 478v , B 478v , C 333v , D 285v , E 267v
Those look normalish, except, if that's the bias voltage at the output tube control grid pins, it may be biased way too cold.
Which could cause crappy distortion - what is the cathode current looking like in each output tube?
Here the question again, moving the tone stack to between V2 triodes and eliminating the cathode follower, would it reduce the harsh gain?
Not likely. Look at the (potentially cold?) bias issue first
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Moving the tone stack at 1st glance looks easy enough to implement without changing much the circuit. Still just an idea,
the second idea is to go twin like fender.
the bias voltage at the output tube control grid pins, it may be biased way too cold.
that is the maximum negative -55
Not likely. Look at the (potentially cold?) bias issue first
We have done it before, bias at like -42 , amp still have lotta distortion and one of the PA tubes did red plate when going a bit above that.
BTW you asked me years back if i had an scope, well now i have, not sure how to use it thou :icon_biggrin:
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what is the cathode current looking like in each output tube?
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what is the cathode current looking like in each output tube?
Meter at 200mV DC
V4 1.5mV
V5 2.4mV
bias at -55.
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I tried @stratomaster suggestion, the resistor from V2a grid to ground, 2.2M and 3.9M the ones i have. can notice a little difference with 3.9 but far from clean.
That forms a voltage divider. You would have heard a cleaner sound with the 2.2M, not the 3.9M. The 2.2M passes less signal to the following grid than the 3.9M.
Your amp is unhealthy. Before you modify it you need to identify the fault.
Edit: reading on in the thread-- your bias is way too cold. You're getting crossover distortion.
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what is the cathode current looking like in each output tube?
Meter at 200mV DC
V4 1.5mV
V5 2.4mV
bias at -55.
Way too cold
you want a good 30mA in each tube - and if an output tube is red-plating at 30mA, you've got a bad tube
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Your amp is unhealthy. Before you modify it you need to identify the fault.
We been through it and no conclusion on that, im willing to give it another go.
Edit: reading on in the thread-- your bias is way too cold. You're getting crossover distortion.
you want a good 30mA in each tube - and if an output tube is red-plating at 30mA, you've got a bad tube
Even with a not so cold bias the amp does distort too much too soon.
One of the reasons i think a bias balance would help. Tubes are not very well match seen so.
Gonna change bias and report back.
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Ok went up with bias
Now V4 at 27mV and V5 30mV
No red plate. (i think back when it did red plate i went a bit too much on bias, if i remember right)
The tone got better but only until the amp start distorting at like volume 3, beyond that its just more distortion.
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Can you post a soundbyte sample of the distortion?
It must be a wiring mistake, bad component or bad tube.
Maybe try tube rolling in the preamp first.
If that doesn’t work, scope the signal chain from front to back with a 1Khz signal at the input (to find where it’s happening)
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Can you post a soundbyte sample of the distortion?
Yeah I'll do that first
Maybe try tube rolling in the preamp first.
that will be the second take
It must be a wiring mistake,
I did check all the connections, the amp works, all controls works,
Now about the wiring it is really old wires cloth type used in the amp, been thinking about replace all the old wires.
bad component or bad tube.
Could be a bad component, some are really old, caps and resistors, we did check the critical caps while back no leak on those,
if it is a tube the swaping may spot something
If that doesn’t work, scope the signal chain from front to back with a 1Khz signal at the input (to find where it’s happening)
How(ground/signal path?) and at what points in the amp to connect the scope(its a really cheap one/probe x1 x10)?
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Don't worry about the scope until you've tried all the other stuff
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Ok here is the sound sample. also in the begin of the video the preamp tubes are visible, both sets of tubes give same behavior with tubes swap. In the video i tried to set both volume controls at about 2, 3 and 4, 1 is clean nice and clear tone, 2ish have a little saturation, beyond it distortion start.
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Are you using audio taper volume pots? Also, I only saw 1 input. Do you have a schematic of how your did the input stages? If you cascaded them (like the hot switch option) then it's performing as intended.
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The input is just like in the schematic in the first post here (https://el34world.com/Forum/index.php?topic=32857.msg364841#msg364841).
Are you using audio taper volume pots?
Im not sure what type they are. those two are really old vintage,
here some images
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Taper absolutely matters on volume pots and correlating saturation and volume settings.
Most use a 10% audio taper. This means that what is normally halfway up on an audio pot is equivalent to just a few degrees of rotation on a linear one.
I recommend you disconnect wires to the pot and measure the resistance from the center lug to each outer lug with the pot set mid way.
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Here is the ohms values in the pots:
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Here is the ohms values in the pots:
You have the wrong pot taper. Linear pots will have next to no adjustment in the clean range. Sub in audio taper and I think you'll get the clean range you're looking for. If not, we can keep troubleshooting
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I got a pair of Alpha pots type A , ill give a try on that and report back.
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Pots replaced, there is good improvement the harsh distortion isn't happening as soon as before. 1 to 4 clean ish, beyond it start to drive.
The amp still very agressive, very grit. I have tried again the mod suggest by stratomaster, 2.2M resistor in the V2 pin 2 to ground, it did help a little more. Amp still need a lot more clean tone and headroom.
Im willing to modify it to achieve the clean clean tone.
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Pots replaced, there is good improvement the harsh distortion isn't happening as soon as before. 1 to 4 clean ish, beyond it start to drive.
The amp still very agressive, very grit. I have tried again the mod suggest by stratomaster, 2.2M resistor in the V2 pin 2 to ground, it did help a little more. Amp still need a lot more clean tone and headroom.
Im willing to modify it to achieve the clean clean tone.
remove the 0.68μF cap from the 2nd stage. You'll probably want to dial in less of the normal channel and a touch more of the high treble channel after doing this to keep a comparable eq.
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remove the 0.68μF cap from the 2nd stage.
Would that be C1b(V1) or C4b(V2) ?
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C1b is the first stage. You just put them in parallel.
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C4b is now out of the circuit, the bright channel now is much more clear and only start drive a bit after 5. could have a little more punch.
The 2.2m resistor is still in place.
The normal channel is too low frequency, bassy and dirty, same as before.
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Cool. Now we're getting somewhere.
Take the 2.2M out and swap C2 and C3. Just remove both and exchange their places. Put the 0.0022 on C2 and the 0.022 on C3.
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With caps swapped, bright channel got punchier(i like it) but lost clarity, no so clean anymore, i guess the ideal value somewhere between the 2 caps but going to the side of .022
Normal is still way too bassy. Maybe it is that cathode cap 330 too much?
Tested with and without 2.2m resistor, with still better, without gets dirty grit tone.
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With caps swapped, bright channel got punchier(i like it) but lost clarity, no so clean anymore, i guess the ideal value somewhere between the 2 caps but going to the side of .022
Normal is still way too bassy. Maybe it is that cathode cap 330 too much?
Tested with and without 2.2m resistor, with still better, without gets dirty grit tone.
The complaints you have about this amp are what makes people so attracted to it. The early breakup, gritty and bright lead channel and bassy normal channel are what made this circuit legendary. The way I like to use this circuit is jumping the two, turning up the lead channel, and just barely feathering in the normal channel to help fill out the low end--essentially using the normal channel volume as a bass control.
I think you'll benefit from a pre phase inverter master volume--which is just a 1M to ground off the treble control with the wiper feeding the input cap to the phase inverter. A great deal of the breakup happens in the phase inverter (which most people enjoy), and the master volume reduces some of that breakup (which is why people didn't like this type of master volume in this circuit).
The input cathode cap is large on the normal channel, but you won't notice much of a reduction in bass content until you reduce it to about 3.3-4.7μF. You can try reducing this to customize the sound, but I think you're better off just running that channel at lower volume and getting much of the sound from the lead channel
Beyond that, you can absolutely modify the circuit to your liking. However I think you're better off with another circuit like an AB763 type.
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The way it works is indeed very interesting, yet the amp does break up too much for my style, I'm used to the clean and big headroom of Fender like amps.
Beyond that, you can absolutely modify the circuit to your liking. However I think you're better off with another circuit like an AB763 type.
You are probably right, that was my second though to modify the preamp to some Fender circuit. I'm doing it, :icon_biggrin: i thinking a twin like , have any suggestions ?!
Above all I really appreciate the time and effort helping me, the fact that super early drive been solved is a game change, thanks a lot !! :worthy1:
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The pre phase inverter master volume is my recommendation. Try that before anything else. You can even use one of the pots you removed from the amp earlier to quickly test if you like what it does.
You can also try making R16 47k-56k and leaving the coupling caps reversed as they are. This will shift the eq of the amp more towards treble than bass and low mids. This might be the clarity and bass reduction you're looking for.
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Tried the 56k I did shift the eq a bit the normal channel is more pleasant now, but still mudy very low.
I'm starting to realize this circuit is not for me. Too much grit and break up.
Sounds like a good idea to change the preamp to a Fender twin like. i got a couple of concentric pots for the tone stack, so avoiding to drill more holes in the chassis.
I got this schematic I've modified while ago, its just a draft, values may be wrong, and some more tweaking is probably needed, also mid control missing in one side, tell me what you think:
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What taper are you using on your bass pot? My experience with the circuit is the lead channel is very bright and brash and the wooly warmth of the normal channel is a welcome contrast and very useful for blending--again at very low levels.
The plexi in stock form is capable of beautiful clean tones with some careful tweaking, especially jumpered. It's just a few components different from a tweed Bassman after all.
Try the master volume before completely redesigning the amp.
You can put a 2.2μF on the normal channel cathode to decrease the low end further or reduce the coupling cap even more.
You can replace the 470k mixers with 220k-270k for lossier mixing without abandoning vintage inspired values. This will affect the bright cap performance.
You can add 22k resistors to the plates at the socket for a split cathode gain reduction.
Lots you can do before a major redesign.
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What taper are you using on your bass pot?
1mA alpha one side read 142 and the other 926, across the pot 1064
Try the master volume before completely redesigning the amp.
Have not tried that yet, I dont want to add another pot to the chassis, the faces finishing is very fine.
You can put a 2.2μF on the normal channel cathode to decrease the low end further
Done that with good result, normal is now way more pleasant than the original setting.
reduce the coupling cap even more
I does feel like could change that a little, to what value thou?
You can replace the 470k mixers with 220k-270k for lossier mixing without abandoning vintage inspired values.
Done, the two resistor in the amp have drifted in value, they were close to 470k back when the amp was built,
now they read 487k and 509k.
I found two resistor reading 255 ish, now in place (note: the 2.2m resistor still in place)
This will affect the bright cap performance.
It did, the bright channel got sharper now.
You can add 22k resistors to the plates at the socket for a split cathode gain reduction.
I didnt understand how to do it.
I got a questions:
The resistors in the PA tubes(R31,R32) screen were originally 1k and were replaced with 2.2k in one of the atempt to reduce the distortion, should go back to original value or what value would best fit for a clean tone?
The OT is ultralinear !! Could those taps be used for cleaner tone ?
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If you like the normal channel now, then there's no need to reduce the coupling cap further. Next standard size down is 0.001μF. This would be pretty unusual for an input stage coupling cap on a clean voiced amp.
You really should try the master volume. You can just temporarily install it for testing purposes. If you like it, then find a place for it on the back of the chassis or even on the bottom. Or if you find a particular setting you like you can used fixed resistors hardwired after the treble pot to create the same voltage divider.
As far as the split plate load, you just disconnect the wire to the plate at the tube socket and add the resistor there just like you would for a grid stopper. You're just squeezing a resistor in between the wire and the socket. 22k might be a bit high, but you can experiment once you get the idea.
I doubt you'll hear a big difference between the 1k and 2.2k screen. I'd leave this as is for now.
Leave the transformer as wired. Most of the breakup is in the PI--which is why I'm so adamant about trying the master volume. You can get the amp to have a but more output power, but to reduce the breakup/distortion the MV is really the direct approach.
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I doubt you'll hear a big difference between the 1k and 2.2k screen. I'd leave this as is for now.
What is the range for the resistor?
Most of the breakup is in the PI--which is why I'm so adamant about trying the master volume. You can get the amp to have a but more output power, but to reduce the breakup/distortion the MV is really the direct approach.
The breakup still too early about 5-6 volume, ill try the volume thing.
Does the position of the presence control have anything to do with it? Its in the PI
When turning up the volume at 4 and above the amp sounds like a person trying to speak with mouth full of food,
that is what i mean by clarity, it sounds very grainy
I did read about the V2 cathode follower been a source of the distortion, what can you say about it?
bring me back to the idea of the modded schematic, tone stack between V2 triodes follow the schematic where i got the idea
Bright channel is too bright very sharp after the mix resistors change, harsh on ears, im guessing changing the .022 C3 cap would bring it down ?
I'll try the plate resistors, which plates?
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I doubt you'll hear a big difference between the 1k and 2.2k screen. I'd leave this as is for now.
What is the range for the resistor?
There is no range for leaving it alone. 1k is a popular value for EL34 screens. 2.2k will give the screen a good bit more protection and result in a touch more compression and slightly lower power. You may be able to measure this, but I doubt you'll hear it.
Most of the breakup is in the PI--which is why I'm so adamant about trying the master volume. You can get the amp to have a but more output power, but to reduce the breakup/distortion the MV is really the direct approach.
The breakup still too early about 5-6 volume, ill try the volume thing.
Does the position of the present control have anything to do with it? Its in the PI
The presence control is in the negative feedback loop, and it absolutely affects breakup. Play around with the settings and how it interacts with the treble and volume controls. You can get cleaner sound by turning the presence down and treble up.
When turning up the volume at 4 and above the amp sounds like a person trying to speak with mouth full of food,
that is what i mean by clarity, it sounds very grainy
I did read about the V2 cathode follower been a source of the distortion, what can you say about it?
It has been shown to be a source of compression and it reduces the insertion losses of the tonestack. You can try bypassing it for experimentation purposes, but again you're looking everywhere but where the breakup you don't like is happening. Put in the pre PI master.
bring me back to the idea of the modded schematic, tone stack between V2 triodes follow the schematic where i got the idea
Bright channel is too bright very sharp after the mix resistors change, harsh on ears, im guessing changing the .022 C3 cap would bring it down ?
No. Coupling caps act like a high pass filter on combination with the plate impedance. Coupling caps determine where the bass roll off occurs and do not directly affect the high end (ignoring phase effects). I'd be inclined to just remove the bright cap and try it that way. Removing the bright cap on a plexi is a popular mod.
I'll try the plate resistors, which plates?
Any stages where you want to reduce gain. I'd do the stage before the cathode follower first because it affects both channels with less work. For even less work and more gain reduction you can try a 12AU7 in V2.
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The resistor to the plates of V1 did work well with the bright channel, volume goes up to about 6 no break up,
22k feels like is a bit too much, ill try bring it down a bit, 18k maybe 15k
not so much with the normal, it still mudy. cathode cap change maybe give it a bit more life?
The master volume did about the same as the plate resistor, tested with both together and isolated.
It does clear up the tone further, i rather keep the MV out thou.
Question about NFB , as far as i know changing it can give more headroom and clean tone, can that be done?
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What an interesting journey. I think you are almost there. You started building a Marshall Plexi but many mods later you still do not like the sound. Been there...
Your ideal seems to be clean sound overall until at least half volume-especially clear bass, no mud allowed!
Sounds like you want a stock Fender Tweed Bassman: Less preamp gain, less midrange, and cleaner feedback circuit. You can keep the EL34 tubes if you don't have a pair of 5881 tubes. You may prefer that sound anyway. The proper components should fit right on your current turret board. Way faster than doing Plexi mods one at a time :lipsrsealed:
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The 5F6A is voiced darker than his amp currently is, where he describes the normal channel as still too muddy.
It also uses a 12AY7 in V1, which is a huge reduction in voltage gain. He described the 100k/22k split plate as too much reduction, so the AY would definitely be too much for him.
He could potentially adopt the NFB/presence circuit to a bit of success, but on all other counts I think a mod to Tweed Bassman specs is a few steps back from his goal.
Uki, I have a feeling you incorrectly installed the master volume after the treble. This is the solution, despite your hesitancy. Try this instead. Put a 220k resistor between the wire at the treble pot wiper and the lug. Then put a 1M to ground at the input side of the 0.022μF cap at the PI. This will emulate a prePI master volume set back a notch or so with no holes drilled. Alternatively you can fit a trim pot to the treble pot and just dial in the amount of signal attenuation you want.
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I stick by the Tweed Bassman circuit because it has great headroom and is not too bassy if the Bass control is properly used.
My other reason is that OP started with a vague schematic and has since made many changes but only documented them verbally. It is not clear which changes were later removed. Using a known circuit such as the Hoffman 5F6A would allow the forum to communicate clearly what the current circuit is and which mods are needed
I whole-heartedly agree that a pre-pi master is needed regardless of his decision on what basic schematic to use. Both amps have plenty of gain available, even with a 12AY7. The master volume is needed to get some hair without blowing out windows.
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where he describes the normal channel as still too muddy
Indeed it is too dark, i though about adding a bright cap like in the bright channel but with a value that doesnt go as bright as the bright channel, would that work? what value?
He described the 100k/22k split plate as too much reduction, so the AY would definitely be too much for him.
I did set it to 18k, sounds good, punchy and clean until volume 6 (i dont have a AY)
He could potentially adopt the NFB/presence circuit to a bit of success
Reducing the resistor there will give more clean tone and headroom yes? 22k ?
Uki, I have a feeling you incorrectly installed the master volume after the treble.
I might have :icon_biggrin: here how i did it, output of trible to one side of the MV, wiper to cap before PI, the other lug to ground. It did increase and decrease volume.
Try this instead. Put a 220k resistor between the wire at the treble pot wiper and the lug. Then put a 1M to ground at the input side of the 0.022μF cap at the PI. This will emulate a pre PI master volume set back a notch or so with no holes drilled. Alternatively you can fit a trim pot to the treble pot and just dial in the amount of signal attenuation you want.
I'll try it as well.
I started yesterday making a schematic with all the changes, so we can keep track on whats going on.
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where he describes the normal channel as still too muddy
Indeed it is too dark, i though about adding a bright cap like in the bright channel but with a value that doesnt go as bright as the bright channel, would that work? what value?
Because of channel interaction a bright cap across the mixer for one channel becomes a "dull" cap for the other channel. This is part of why the normal channel is lacking top end. If you're going to experiment with a bright cap on the normal channel put it on the pot like on the 5F6A.
He described the 100k/22k split plate as too much reduction, so the AY would definitely be too much for him.
I did set it to 18k, sounds good, punchy and clean until volume 6 (i dont have a AY)
He could potentially adopt the NFB/presence circuit to a bit of success
Reducing the resistor there will give more clean tone and headroom yes? 22k ?
Yes, at the expense of some dynamic response and "feel". Try it, you might like it.
Uki, I have a feeling you incorrectly installed the master volume after the treble.
I might have :icon_biggrin: here how i did it, output of trible to one side of the MV, wiper to cap before PI, the other lug to ground. It did increase and decrease volume.
That sounds right. I'm surprised you didn't notice the significant decrease of breakup by just turning it down slightly.
Try this instead. Put a 220k resistor between the wire at the treble pot wiper and the lug. Then put a 1M to ground at the input side of the 0.022μF cap at the PI. This will emulate a pre PI master volume set back a notch or so with no holes drilled. Alternatively you can fit a trim pot to the treble pot and just dial in the amount of signal attenuation you want.
I'll try it as well.
I started yesterday making a schematic with all the changes, so we can keep track on whats going on.
I hadn't realized you put C4b back in. That will definitely increase the breakup. If you want to keep the effect on the tone of that cap but reduce the boost, you can try adding a low value (2.2k or so) resistor in series with C4b. Or you can reduce the 2.2M resistor from the grid to 1M. Since you reduced mixers, this is needed anyway to get a similar effect as before the change.
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I hadn't realized you put C4b back in.
Ugh i forgot about that one, didnt remove it actually, ill check that out
Or you can reduce the 2.2M resistor from the grid to 1M. Since you reduced mixers,
this is needed anyway to get a similar effect as before the change.
I was wondering about it, if could reduce it further, ill try it too :icon_biggrin:
Question about the PI , the 2 smaller resistors have smaller values than some Fender amps,
here we have 470 and 10k, some fender amps have 820 and 22k, is that related to the early breakup ?
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Question about the PI , the 2 smaller resistors have smaller values than some Fender amps,
here we have 470 and 10k, some fender amps have 820 and 22k, is that related to the early breakup ?
Here's a good overview of the long tail pair.
https://www.valvewizard.co.uk/acltp.html
Those resistors set the bias and balance of the PI. The 470 does bias hotter, but the designs you're comparing to employ a 12AT7 instead of the 12AX7 of Bassman/Plexi--so it's not apples to apples.
You can try a 12AT7 in that spot for experimentation. It won't hurt anything.
I'd recommend reading up on that page (and a deeper diver in the Aiken amp white papers) before you go changing values in the PI. Or if you insist on messing with it to pick a tried and true design like the BF or later SF-- but mind the voltages and tube types employed.
I think you're better off limiting gain into the PI than changing its design, though.
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The 2.2m resistor is now 1m, not much change, tone is clean ish, still grit, grainy, amp is somewhat clean until 4-5 depending on guitar pickups, the cleanest set on guitar still too much grit on amp, bridge humbucker is pure rock'n'roll !! :m7 too much :huh:
The C4b was out of the circuit already, I forgot to remove from the schematic, now updated in the post #53 (https://el34world.com/Forum/index.php?topic=32857.msg365229#msg365229)
NFB resistor down to 22k , not much change with it either. Try 10k ?
I'm thinking if there is some not so good component in the circuit, there are a number of super old resistors and caps.
I'm still thinking about to move the tone stack between V2 triodes to see if it will take out that grit.
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I can assure you there are drastic changes in gain and tone with both the reduction of the 2.2M to 1M and the reduction of the NFB to 22k.
Did you try removing the bright cap from the lead channel yet? That will both reduce upper frequencies in the lead channel and add high end to the normal channel.
Did you try bypassing the cathode follower? This will increase the tonestack related insertion losses and reduce gain into the PI.
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I can assure you there are drastic changes in gain and tone with both the reduction of the 2.2M to 1M and the reduction of the NFB to 22k.
I did notice a good gain(drive) drop overall
When both volumes go up amp start going wild :icon_biggrin:
Did you try removing the bright cap from the lead channel yet? That will both reduce upper frequencies in the lead channel and add high end to the normal channel.
Gonna try that next, bright is too sharp and i notice now normal can get better tone increasing treble control. Mid and Presence makes the amp go wild, specially together
Did you try bypassing the cathode follower? This will increase the tonestack related insertion losses and reduce gain into the PI.
I'm not sure how to do it :think1: jump from V2 grid to the input of tone stack ?
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When you say "going wild" do you mean overdrive or is there a squeal/oscillations?
The pre phase inverter master will enable you to turn up the treble and turn down the gain into the PI. Again--it's the answer to get what you want. You should temporarily install one and play it for a week or two to get familiar with how it interacts with gain and EQ.
See the attachment for a quick way to bypass the cathode follower.
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When you say "going wild" do you mean overdrive or is there a squeal/oscillations?
Overdrive :m11
I did check again the NFB resistor, it was a bit too hight, one of the really old Rs, replaced for a 22k right on spot,
and tone did improve good amount
I'm getting some bad noise on the speaker, i was checking it and looks fine, checked for loose parts all looks good solid,
bad power tube maybe? :sad2: Gonna watch the PA tubes while playing
See the attachment for a quick way to bypass the cathode follower.
Is this jump bypassing only the sencond half of V2 ?
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Is this jump bypassing only the sencond half of V2 ?
Yes. That's the cathode follower.
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Have done the cathode jump, no improvement, i totally disconnected the 2nd half of the tube.
the amp did improve a lot since we started thou and it is way louder !
I think one of the power tubes are going bad, when playing soft amp does ok, but if I play hard amp does some farting sound.
When tweaking bias some years back, went too far without knowing it and one of the tubes did red plate,
i didn't notice right the way, and that one tube had some sparks inside... so probably that...(i could be wrong thou)
maybe the screen voltage is too high for EH tubes ?
But now, I'm not sure which one is it thou :dontknow:
I was thinking the spring coils holding the tubes were the cause of noise, but after putting it aside amp sound the same,
I got another pair of EL34 but totally unmatched. that is why the questions about the bias balance/adjust earlier.
(bough those two cos was very cheap, but not as pair)
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You again should have had a significant drop in gain and an increase in clean headroom. I think too much time is passing been comparison points. Perhaps record them to get a more accurate comparison.
Plus it's hard to judge if the amp isn't healthy. You need to get the power tube situation figured out before proceeding.
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You again should have had a significant drop in gain and an increase in clean headroom.
Yes indeed !
I think too much time is passing been comparison points. Perhaps record them to get a more accurate comparison.
You got it !
As you can see the amp got good improvement yet still really agressive :icon_biggrin:
Plus it's hard to judge if the amp isn't healthy. You need to get the power tube situation figured out before proceeding.
The fart sound is solved, the 22k NFB resistor was too low, i went back to the 30k ish one.
Is the screen voltage too high? Data sheet says 425v max, it is over the chart !
Side note: I use a preamp inside my guitar, it boost the signal, it isn't engaged in those videos, amp goes way more wild with it on.
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Back on it
I've check the amps on the other two PA tubes(Mullards) that i have, they are very unbalanced, 37mV and 28mV.
only now crossed my mind to see if it follow the tubes when swapping them, I think it probably will,
the tubes that were in the amp before are EH.
(I can't afford at the moment to buy another pair, tubes are way too expensive (https://www.multcomercial.com.br/catalogsearch/result/?q=el34) right now)
Then to use those two tubes amp need separated bias adjust per PA tube, or a balance/adjust.
Questions:
Which one would be better for setting the bias: adjust/adjust or balance/adjust ?
The voltage(478v) after the choke is the same as before the choke, is it normal ?
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Questions:
Which one would be better for setting the bias: adjust/adjust or balance/adjust ?
The voltage(478v) after the choke is the same as before the choke, is it normal ?
Either one will get the job done. There are several examples of each on this forum.
Yes. The purpose of the choke is to create an LC filter to minimize noise and simultaneously minimize voltage drop vs plate. If you had a benchtop meter you'd see the drop in the 2nd to 3rd decimal places.
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...
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I've made a small board for the bias circuit, balance/adjust.
I did follow the values like in the picture attached, or should i follow the values posted by Willabe?
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They'll both work. Pick one. Adjust as needed if needed.
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The BIAS is in place, the range changed, now goes between -10v to -39v
Does it need more negative voltage ?
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I personally don't like seeing anything closer to 0 than -20v unless the situation calls for it.
Please post a schematic of what you built.
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I personally don't like seeing anything closer to 0 than -20v unless the situation calls for it.
That is what i was thinking, before it was something around -20ish to -55ish
Here is the whole bias section.
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Have you tried a 12AT7 in the PI position?
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Here is the whole bias section.
I'd replace that 220k with a 100k and see where that gets you.
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Have you tried a 12AT7 in the PI position?
I don't have one :sad:
I'd replace that 220k with a 100k and see where that gets you.
I'll try that !
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@stratomaster The 100k resistor did the trick!! :worthy1:
Now bias range is -20v to -70v
I did set the bias voltage to -46v and 27mV on each PA tube cathode , just to check if the circuit is working
what is the ideal setting for EL34 ? Tubes now are Mullard
Tomorrow ill be able to do a sound test
Schematic updated in the post #53 (https://el34world.com/Forum/index.php?topic=32857.msg365229#msg365229)
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Ideal setting doesn't exist. There's a range. And that's not exclusive to EL34s.
For longevity I'd bias as cool as possible, probably 50-55% dissipation, before the crossover notch becomes too obtrusive. Hook to a scope to verify. If no scope, bias to 50% and listen/play for a while. If it sounds good, then leave it.
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probably 50-55% dissipation, before the crossover notch becomes too obtrusive.
:think1: I'm not sure what you mean. I think i can figure out the watts output amp is generating, if that is what you mean.
Hook to a scope to verify. If no scope, bias to 50% and listen/play for a while. If it sounds good, then leave it.
I got a lil scope very simple but only can handle like 30v tops, never really learned how to use it thou,
except to check electrolytic caps :icon_biggrin:
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https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
Read this.
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I got some time today to play for about 30mins, the amp bias still as posted above, it does sound much, much better with the Mullards! :m8
Now it is possible dial some clean tones, the amp does go on overdrive at about 5-6, depending on the controls setting, earlier.
I can't tell difference on volume after 6, feels like it stay the same, just adding drive.
I got another amp that is able to stay clean at volume 7(gain/master controls), it have 3 preamp tubes 12ax7s and 4 el84s,
someone on some forum said it is sort of a Peavey classic 50 or something like that.
Not sure if possible to achieve clean tone on that much volume with the Plexi.
Now something that is bugging me, the screen voltage, 470ish way above specification, which is max 425v,
would that shorten the tube life? I wonder if that is the cause of the aggressiveness of the amp as well.
good thread here (https://www.diyaudio.com/community/threads/typical-plate-voltage-for-el34.82623/)
I don't think i have mentioned before, but the OT have UL taps, and it does have less voltage than the outer taps,
could that be one way to drop the screens voltage? Diagram of the PT attached.
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Now it is possible dial some clean tones, the amp does go on overdrive at about 5-6, depending on the controls setting, earlier.
I can't tell difference on volume after 6, feels like it stay the same, just adding drive.
This is typical tube amp behavior and not exclusive to the plexi
If you have a scope you can check for output power at clipping onset. If it's in the 45w ballpark then there's no additional clean volume to be had.
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If you have a scope you can check for output power at clipping onset. If it's in the 45w ballpark then there's no additional clean volume to be had.
I do but not sure how to do it
I did this: cathode resistor mA times plate voltage. 478 * .027 = 12.9 , almost 13 watts per tube, is that right?
I didn't check the plate voltage after changing the bias circuit thou.
What about the screen voltage?
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I do but not sure how to do it
I did this: cathode resistor mA times plate voltage. 478 * .027 = 12.9 , almost 13 watts per tube, is that right?
I didn't check the plate voltage after changing the bias circuit thou.
What about the screen voltage?
How to measure power:
You will need to recheck your plate voltage as it changes with the bias. But you're near enough to 50% that I wouldn't worry too much.
Reducing the screen voltage will reduce output power, not increase it. It's not uncommon for tube guitar amps to run the tubes outside of their published limits. If you want to have a heart attack look at how the AC30 treats the EL84s.
I think your amp is behaving normally unless there's an issue with the power output.
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The stock Marshall Plexi and Tweed bassman amps are capable of playing VERY LOUD AND CLEAN if the controls are set properly. Your amp is broken if it will not do that.
Turn the Bass control to 1. Is the mud gone? Does it play loud and clean? No? Then it needs repair, not tweaking. I'm sorry :sad2:
Repair will be very challenging because you do not have a proven working schematic and proven good parts. You will need to use an oscilloscope to find the circuit locations that are adding distortion.
If that is not possible then I recommend rebuilding with a proven schematic--NO MODS YET! and new parts. This may include the transformers if you cannot prove that they are suitable for this project. OUCH!
Good luck :headbang:
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Reducing the screen voltage will reduce output power, not increase it. It's not uncommon for tube guitar amps to run the tubes outside of their published limits. If you want to have a heart attack look at how the AC30 treats the EL84s.
Ok so that is normal on guitar amps.
I think your amp is behaving normally
It seen like so
I did move the bias a bit sounds even better now
tube base is getting hot, can't touch too long, no red plate, all seen normal,
played for about 20mins sounds really good!
here is the voltages
V4 plate to cathode 460 - bias -45 - current .032
V5 plate to cathode 457 - bias -42 - current .032
also i did the measurement like in the video, not sure if i did it right thou, i got a video :icon_biggrin:
in the video open the normal and bright volumes one at a time.
@stratomaster thanks so much for carrying me through this!
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You have to turn it up until the peaks of the signal flattens slightly, just like that the video does.
Your traces show a bit of oscillation distortion, too. The amp is still unhealthy.
If you are turning the volume all the way up on the video and you aren't seeing the signal flatten, turn up the input signal. I tend to use 100-120mV. As it stands right now you're demonstrating less than 1 watt output power.
If you're running into an 8 ohm load you should have about 19V at the dummy load.
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You have to turn it up until the peaks of the signal flattens slightly, just like that the video does.
Oh i thoutgh that was the limit, it was about half way of the volume, since drive start just after 5 i though it was the point,
now that you mention, the wave gets all crazy after that point, not flat but crazier than appear in the video.
Your traces show a bit of oscillation distortion, too. The amp is still unhealthy.
oh dear :sad2:
If you're running into an 8 ohm load you should have about 19V at the dummy load.
Yes 8ohm
Ok here is the whole story of the amp: https://el34world.com/Forum/index.php?topic=23039.msg246988#msg246988
That may help
If you are turning the volume all the way up on the video and you aren't seeing the signal flatten, turn up the input signal.
I'll do it again and report back.
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The output reading got a bit different this time,
I've connect an 8ohms/45w dummy load(it does read 11ohms with the cable to the amp),
that crazy noise in the wave did not happen with the dummy load.
i did notice a tiny sound even w/o speaker, from PA tubes maybe?
Notice the painting of one PA tube V5 did decay more than V4, is it due to the unbalance and setting ?
I did feel the paint smell while playing, the decay seen rather quick although did not play that long, should I lower current in the tubes?
The wave start flatting at about 8v, round at about 7.5v with a small variation between clean and normal channels,
9.7v at max volume, on both channels isolated, and together it drops to about 8.9v
Here some pics:
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If you're getting flattening at 8v, then you're only getting 8 watts out of the amp.
You've got a good bit of work to do before this amp is ready. It should be deafeningly loud in the room.
Annotate the schematic with DC voltages especially the plate and screens of the EL34s.
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Done, I've notice a little drop on all voltages, probably cos the time of the day, more usage of electricity.
Schematic updated on post #53 just as before, link here (https://el34world.com/Forum/index.php?topic=32857.msg365229#msg365229)
470R is on the hot side for a 12AX7 LTP. You want more like 820R to 1k. If the LTP bias is too hot, if can muck up the LTP duty cycle, resulting in red-plating on one side of the output stage under large signal conditions
this was a post on another thread about this amp, only now I'm starting to understand what he said, yes one side runs hotter than the other.
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The easy test if one side is significantly hotter than the other is to measure the voltage across the cathode resistor under load. You can even measure across the cathodes. The goal is for there to be a 0 measurement. If it's positive then the side you have the red lead on is running hotter. If it's negative then the other side is running hotter. This measurement will also tell you how much higher.
I think you have a different problem, though. But it's easy enough to check.
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You can even measure across the cathodes. The goal is for there to be a 0 measurement. If it's positive then the side you have the red lead on is running hotter. If it's negative then the other side is running hotter.
I did the reading across the cathodes, red lead on V4, with 1khz signal, it start at zero, and goes negative as the signal is injected about -2.3mV
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That's nothing. A few mA difference is within matching parameters. You can tweak your balance pot if you really want it to be "matched" under load, but I think your problem is elsewhere.
What is your plate and screen voltage under load?
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Slowly turn bias from -53V toward -30V. Watch the milliVolts on the 1 Ohm resistors. For now, stop at 20mV (=20mA), which is on the cold side, but *safe*, and should play OK with slight hoarseness in note decays.
35mA may be a good final goal, but I would play it a long long time at lesser current to shake all the bugs out. Those vintage parts may resent being re-awaken after a decade of sleep.
I did follow his advice and turned down the bias
Here is the plate and screen voltages under load
end bias.
V4 plate 390v screen 357 bias -43 .022mV
V5 plate 390v screen 363 bias -47 .021mV
I did notice some variation while reading the screens, like going up and down then stable, i did use 2 meters at once, one on each tube.
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You changed the bias so now we can't compare directly idle to load.
Please try not to add more variables during troubleshooting.
Leave the bias as is and remeasure the amp DC voltages, and please let me know what the load was that you used for the measurements you provided on this post. Thanks.
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You changed the bias so now we can't compare directly idle to load.
Please try not to add more variables during troubleshooting.
Sorry about that
i put the bias back to previous values, so now we are near to previous reading, all readings redone as follow
New readings for DC , load 1khz
volumes set to 50% on all readings
bias:
V4 -47 .029mV
V5 -44 .029mV
no load:
V4 plate 460v screen 452v
V5 plate 460v screen 454v
with load:
V4 plate 451v - screen with one volume either, 440v; both volumes 405v
V5 plate 451v - screen with one volume either, 442v; both volumes 407v
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That screen is sagging more than I'd expected, but not enough for your output to be 20% of what it should be. I was hoping for an obvious voltage difference.
Without your amp on my bench I'm at about the limit of what I can do.
The only other suggestion I have is to signal trace with the scope at each grid and plate (except for the cathode follower, do grid and cathode). My only apprehension is I'm not sure that little scope can handle the DC voltages.
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The only other suggestion I have is to signal trace with the scope at each grid and plate (except for the cathode follower, do grid and cathode). My only apprehension is I'm not sure that little scope can handle the DC voltages.
Here is the scope manual, the probe I have is x1/x10 (gonna order a x100), maybe i can make a signal tracer got an schematic.
The parts of this amp, transformers, and some components, were salvaged from an old hi-fi amp, here (https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697), it came without power tubes and rectifier, maybe the OT is under rated, and it is the cause the amp isn't as loud as it should be? Follow a photo of the amp circuit where the parts came from
I'll check again all parts values, and connections, maybe i missed something.
I appreciate very much the time and effort :worthy1: the amp is much better than ever!
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The transformer very well could be the issue.
It's not the probe I'm concerned about. You don't need a 100x probe. It's the input capacitor on the scope itself for AC coupling.
Looks like it might be just some small ceramic caps at the input.
I wouldn't expose those to much DC at all.
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Lets say it is the transformer, could the amp still be put to use, any risks there?
One thing i've notice, volume doesn't increase after 5, only more overdrive or distortion.
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Lets say it is the transformer, could the amp still be put to use, any risks there?
One thing i've notice, volume doesn't increase after 5, only more overdrive or distortion.
You can use it, but if it's the wrong transformer then you'll eat tubes, so eventually the cost to keep running it vs just buying the right transformer stops making sense. If the transformer is the issue, once you just pony up for the right part you'll wonder why you waited so long.
You can take out the transformer and do voltage tests to determine the turn ratio and make an educated guess as to what it is--if you're unsure. I haven't read your other thread as it's information overload, and I was wanting a fresh perspective. I'm assuming no one in your previous thread caught that you were using the wrong pot taper, for example.
The behavior you've described about getting more distorted instead of louder is normal and desirable. It's what keeps tube amps around.
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I don't remember how did i came about with those numbers thou
but here it is the turns ratio:
32:1
25:1
19:1
32 * 32 = 1024 1024 * 4Ω = 4096Ω
25 * 25 = 625 625 * 4Ω = 2500Ω
19 * 19 = 361 361 * 4Ω = 1444Ω
32 * 32 = 1024 1024 * 8Ω = 8192Ω
25 * 25 = 625 625 * 8Ω = 5000Ω
19 * 19 = 361 361 * 8Ω = 2888Ω
32 * 32 = 1024 1024 * 16Ω = 16384Ω
25 * 25 = 625 625 * 16Ω = 10000Ω
19 * 19 = 361 361 * 16Ω = 5776Ω