Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on January 25, 2015, 12:42:31 pm
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On my breadboard i put together the beginning of an amp idea I had. It is using the hoffman AC 30 power and output section and a preamp similar to a 2204. So it is using two full 12ax7 in the preamp instead of just the one and half like in the top boos channel of the hoffman ac30. I also implemented a full tone stack rather than a treble and bass only.
I got it all thrown on the breadboard and turned it on. No sound at all. Tubes light up. So far, the only thing I have checked is to make sure that the transformer is putting out the 550v that its suppose to. I also checked rectification (solid state) and I am getting about 400vdc. Before I go into messing around any further, I want to see if this is my problem. I had an old 30 watt OT transformer from a jtm30. I had sort of discussed this long ago before my build but wanted to go ahead and try it since i didnt get an answer if it would ACTUALLY work or not. So the jtm30 uses two 6l6 and this is 4 el84. Would I at least get sound using this OT or would it just plain not work? At least no smoke, smells, or blown fuses!! YAY.
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So the jtm30 uses two 6l6 and this is 4 el84. Would I at least get sound using this OT or would it just plain not work?
It will work.
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Thanks Sluckey. I'm going to guess its a tone stack issue. I'll start there.
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I'm going to guess its a tone stack issue. I'll start there.
You can try and just bypass the TS temporarily with an alligator clip jumper to see if you get sound then. Might have to lift the TS ground also incase you have a short in the TS wiring.
Brad :icon_biggrin:
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I would start by measuring voltages on ALL pins of ALL tubes. Also measure voltage on every B+ filter cap.
Then I would touch the end of the cap that feeds into the PI with my finger. If you get a buzz thru the speaker, then look at the preamp/tonestack stuff. If no buzz, then troubleshoot the PI and power amp.
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im wondering if i might have made a mistake in filament wiring. Looking at the heater diagram of the hoffman layout, i dont understand it. V4 shows the number 4 on both sides of pin and v5 shows number 4 on one and number 5 on other
on the schematic, heater wiring is odd to me
http://el34world.com/Hoffman/files/Hoffman_AC30.pdf (http://el34world.com/Hoffman/files/Hoffman_AC30.pdf)
To me it just looks like an error on the schematic
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Your voltages will tell you. Measure your AC voltage across the filament pins of each tube, not from each fil. pin to ground.
If all tubes light up, I'd think they're wired right.
If you're getting no sound at all, and your heaters are right, it's a missing connection most likely. Easy to miss one! Post your voltages measure like Sluckey says and he's most likely be able to tell you exactly where to look.
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Got the readings.
keep in mind...this is not my pre amp. i have 470k plate resistors. full tone stack. and Im using 2 full 12ax7's before phase inverter.
Im not getting any noise in speakers at all. not even a hum. touched the cap going into PI and no noise still.
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57 volts is WAY too much for the cathode of those EL84s. Should be more like 15 to 20 volts. Make sure that 50Ω/10W resistor is really 50Ω.
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Well....there was one problem. Didnt have the 50/10w and 220uf grounded..... got some noise now.... SOME CRAZY NOISE. No idea where to start from here ha.
None of the knobs seemed to affect the noise, however the cut knob did seam to change the tone of the noise so I guess that one might be ok.
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Pull all preamp tubes except the PI and the four output tubes. Does the noise go away? If you touch the input cap to PI do you hear a buzz in the speaker?
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Pull all preamp tubes except the PI and the four output tubes. Does the noise go away? If you touch the input cap to PI do you hear a buzz in the speaker?
It got rid of the CRAZY noise. Still has a hum to it. Kinda loud. But that could be the open chassis in combo with my bad grounding here and heater wires on the test board. But when I touched that cap, there was a change. it almost decreased the hum.
-----> I take that back. It did not make a change but I did notice noise when I touched that cap. With the MV turned down, had a hum but no rumbling noise. When i turned that knob it all came back. This was with me pulling the 2 preamp tubes but leaving the 4 el84 and PI in the amp.
I disconnected the wire going into the input cap of PI (still have preamp tubes pulled) and I still got very loud hum.
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I wouldn't be too surprised if your supply string is under-filtered. The HT and screen supply power caps are 16uF on the AC30. If I remember correctly they are 50uF on the 2204. Filtering from my experience can significantly impact hum and noise in an amp as well as headroom and the tightness of the bass, etc.
If you have any extra 50uF caps you may try to switch them and see if it effects the noise level.
Thanks
Mike
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I wouldn't be too surprised if your supply string is under-filtered. The HT and screen supply power caps are 16uF on the AC30. If I remember correctly they are 50uF on the 2204. Filtering from my experience can significantly impact hum and noise in an amp as well as headroom and the tightness of the bass, etc.
If you have any extra 50uF caps you may try to switch them and see if it effects the noise level.
Thanks
Mike
I guess the only thing that makes me skeptical of that is, if i disconnect the preamp (which is what has 2204 characteristics) all together and leave just the power amp (hoffman ac30) i still get the noise.
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I should mention that this isn't a working amp with noise. It's an unbearable noise in which I can only keep it on for a few seconds at a time to get readings.
I'm calling it a day but the very last thing I did was:
Pulled first two preamp tubes. Left phase inverter and all el84. Master volume all the way down. Turn amp on. Loud hum. Turn up master volume and unbearable thump thump thump thump repeating on top the hum.
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one of my pre-checks on a new build:
check the grid pin and cathode pin of every triode and pentode's ohm reading to ground. compare to schematic as you go. grids will always have a reference to ground, and the cathode to ground reading should be straight forward. volume and tone stack pots will also have a reference to ground. check them as well.
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May I suggest... Since this is a breadboard project, that you just disconnect the preamp from the PI. Then make the PI and power amp and power supply all work correctly. Once that is successful, connect the preamp and go from there.
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May I suggest... Since this is a breadboard project, that you just disconnect the preamp from the PI. Then make the PI and power amp and power supply all work correctly. Once that is successful, connect the preamp and go from there.
Can I just wire my input Jack to the input cap on pin 2 of PI?
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Make it quiet 1st -- without a preamp, all voltages good, then you could use a cd, mp3 kinda "source" signal. Then you're onto the preamp.
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Make it quiet 1st -- without a preamp, all voltages good, then you could use a cd, mp3 kinda "source" signal. Then you're onto the preamp.
i could just use my guitar still right?
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Make it quiet 1st -- without a preamp, all voltages good, then you could use a cd, mp3 kinda "source" signal. Then you're onto the preamp.
i could just use my guitar still right?
They're mean no; no input jack, no guitar. The circuit as-is shouldn't have hum or noise with just the phase inverter and output stage.
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If I wired and input jack at that point right into the phase inverter wouldn't that be the same as having an FX return?
What I am going to do today is disconnect the preamp 100% and just focus on the power amp and making sure there is no noise when I turn it on. I have a few questions though before I do this.
– my bus bar is one long busbar. Power amp and a preamp both share it. This is how I've done every amp I've ever done but I've noticed on the layout of the Hoffmans amp that the power amp has a separate busbar than the preamp. I have not done and EL 84 amplifier before so I figured it wouldn't hurt to ask if I should change my grounding method for this amp?
-my output transformer is kind of suspended off of the chassis on a block of wood due to lack of space to fit all of my components on my breadboard. Does the metal casing of the output transformer need to be grounded by contact on the chassis? I didn't think of this until now.
I guess I'm just going to try to organize everything a little better and traced down the circuit and check values on every component of the power amp
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Put a jack on the input cap for the PI. You will need a line level signal for full output. CD player output works fine. Your guitar will work also but don't expect it to be loud. It should be clean sounding with no noises, buzzes, hums, etc.
Don't be too fussy about ground busses on a breadboard. Just get everything connected IAW the schematic. Worry about proper grounding techniques when you start to permanently build something in a chassis.
The OT frame does not usually need to be grounded unless the secondary common lead is connected to the frame.
Look at the attached circuit. Make this circuit work correctly before connecting a preamp.
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Sluckey I owe you. Thanks. I'll work on this today since the ice cancelled my outside jobs!
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ok so heres a report. One thing i need to discuss are what wires to keep away from one another. Now I know heater wires....stay away from it all. But I had some crazy noise and as I moved the wires I got rid of it. So right now I have.....just a slight little hiss. And I have a very low volume on my guitar. I am going to try a CD player of some sort in a few.
So any way these pins on my EL84 close together make noise but separated take away the noise.
Pin 2 and 7- now its my understanding that 2 is the input while 7 is the output....and we want those to stay away correct?
This seems to be the main one. Others are just minor hissing. LIke all my number 9's of el84 going into the 100ohm 3 watt...some of those can make a little noise if directly crossing over top and touching the other.
Any other wire routing advice would be appreciated. :icon_biggrin:
[EDIT] I have since added in preamp. The full premp was a no go. I then added only half a triode with volume control. Got sound. Wasnt very loud. When I strum real hard it almost had a sudden burst of volume for a split second. Kinda sounded compressed or like a heavy gate. Im wondering if this could be a result of a loose connection?
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at this point i dont know what i could possibly do different :BangHead:
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You can make sure it's wired according to the schematic. You do have one right? Post your schematic for us to see. We need a little more info than just "a preamp similar to a 2204".
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You can make sure it's wired according to the schematic. You do have one right? Post your schematic for us to see. We need a little more info than just "a preamp similar to a 2204".
I have to back up. Im not ready for preamp yet. Im still getting a screech in my power amp and Im not sure if its wires too close to others or what. The screech changes frequencies when cut knob is turned...and as it should I guess. Im just real tight on space in this section and not sure which wires should stay away from which wires..
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Try and keep wires at 90 degrees to each other as they cross, especially signal wires crossing power wires. Ideally they don't cross but this is the real world. have you got voltage numbers on what you're working with, PI, PA, PS? If so maybe post them, but also every time you set down to "do something", re-measure them, did they change >20%, did they start making the amp bark n howl this time but not last time?
These sorta things and more will help you develop "your method" for chasing down problems easy and efficiently
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Well im getting somewhere. It is the quietest its been....and loudest volume ive gotten with guitar. Only thing is, when i strum.....it kinda krackles. This is just power amp....no pre.
I haven't taken any measurements yet. Im going to do so soon as I just finished moving wires and adjusting the layout a little.
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Im pretty stoked on this one...i figured it out myself. I thought it sounded out of phase.....sure enough. 2 PI wires wire backwards. :icon_biggrin:
I think im ready to pre amp it up!!!!
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This is probably going to be it for me for a few days but heres where I am.
I put the hoffman ac30 together using the Normal channel that only uses half of a triode. I put it together just like the schematic. Not much to it.
I cant turn it up much. At a very tiny low volume it sounds ok but turn it up much at all its very very noisy and screechy. I am using some metal oxide resistors that I got from parts express a while ago when I was working on some crossovers. I know metal film is ideal but could these metal oxide be causing it?
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The top two problems when a new build does not work are...
1. Wiring error.
2. Wrong component.
Other top contenders are poor connections or solder joints, and poor wiring technique.
Can we now assume you have followed Hoffman's schematic, but with normal channel only? Show us some pics. Lots of 'em. Big and clear.
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I have a couple videos of the build on the bread board. The el84 power amp from the hoffman AC30 schematic seems to be working fine. After installing an input jack at the PI cap and plugging in my guitar.... all seemed well.
From there I used the normal channel from the AC30 schematic as well. Just to get a start at my preamp design. In this video you will hear the noises that I am getting. It almost sounds like blown speakers but I tried multiple cabinets. When notes are strummed hard at lower volumes I get the noise. When it gets passed a certain volume it makes the noise with each note no matter how hard you strum.
Video is here. http://rdkelectricalservice.com/playingamp.mov (http://rdkelectricalservice.com/playingamp.mov)
My preamp consists of this chain.
Input jack > - to ground bus and positive to first component, 1m resistor to ground and 33k resistor to input 2. > Cathode of half triode used is 10uf cap and 8.2k resistor (was trying to keep high to reduce gain) > Input 1 to a .047 coupling cap and a 100k plate resistor being fed from one half of the double 8 uf filter cap. > signal exits .047 coupling cap and hits a 100k resistor (i only did this to try and reduce some gain exiting into volume pot to try and fix noise. same noise with or without that 100k) > signal then passes into the right side of a dual 500k volume pot (I only had a dual 500k). the left side of volume pot hits ground while the signal leaves the center. No bypass cap on the volume pot.> from there signal hits a 220k resistor in series with another .047 cap which is the input of the PI.
So...theres not much to this. This could be a goofy thing. Now like I mentioned above, the power amp seems fine to me when played alone. Could the issue still be in power amp? I guess so since there are so few components in the chain of the preamp. But you will have to tell me that. :icon_biggrin: Could I have a loose connection still? Very possible. I tightened down all my connections and ended up over tightening them which lead to many broken leads on my components which I had to re connect. Hope this is enough info and hope it helps. Heres a video of my preamp on breadboard http://rdkelectricalservice.com/testboard.3gp (http://rdkelectricalservice.com/testboard.3gp)
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Cathode of half triode used is 10uf cap and 8.2k resistor (was trying to keep high to reduce gain)
Change the cathode resistor to 1.5K. Any help? Omit the 10µF cap to reduce gain if necessary.
I have audio only for that .3gp file. Can't see any video. Probably a codec thing on my end. Maybe you can convert to a more pc friendly format. Or post pics. I like pics.
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I took off the cathode cap and changed the 8.2 to a 1.5. Still got it. I have a theory and I wonder if this could possibly be. When something in the wiring in a home has a loose or broken connection, ill use the 240v mains coming into the house as an example. Ive been in a situation where a customer is losing one leg of the 240 coming in their house. Saying that only half their house works. If its a loose connection at the meter base or the crimp connections at the service or sometimes even a broken wire thats barely making any contact at all, you can turn on the stove burner (since it requires both legs) and that initial jolt of current kind of helps the loose connection or broken contact make a connection and then the parts of the house that were out will suddenly work again (temporarily of course). Now I dont know if this is at all what I am experiencing but it at least SEEMS that my signal flow is being affected in a similar manner. In the video I posted, i played at a very low volume and it seemed ok. I turned it up a bit and it started to sound like a blown speaker. What I didnt do in the video (which I just did when I swapped the components you asked) is turn it up a good bit and see what it does. When I turn the volume up passed maybe 2 or 3 it seems to not give me any signal at al unless i strum the string kind of hard, in which case i get an initial note through the speaker but its a split second......It sounds as if I am playing a 16th note through the speaker....but my guitar is actually sustaining that note. So its an initial jolt of a note then nothing. Something I also have noticed is after I turn the amp on and let it warm up.... as soon as I turn on the standby switch I get an initial kind of popping sound but more dirty. Kind of the same sound I get when I strum those notes on the guitar with out the guitar note. I guess realistically I need to check my connections again but this is only based off my theory. I dont want to make any assumptions and do work I dont have to. Im just going to take your direction. Here are some photos of the build.
This is a photo of the PI input and the preamp. I have a bunch of unused components on the board as I originally had my full preamp in but disconnected it. Now it is only the first half of the triode. The second half is completely disconnected.
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Here are a few more. The four 1.5k resistors tied to the red wires are pin2 of the el84's.
All the cathode of the el84 are tied together into one wire then connected on the lugs.
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WOW! That's quite a mess you have there. I don't know where to start.
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:dontknow: ehh....yea pretty much. I'll clean it up this week and try some photos again.
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I think the problem your having is because on your bread board you have the tube sockets too far away from the barrier strips. Because of this you are dealing with feet of hook up wire instead of inches of hook up wire.
Here's a link for a bread board that SILVERGUN made and has very good pictures of it along with pictures of DL's zilla board, which was SG's inspiration/example.
Thread link;
http://el34world.com/Forum/index.php?topic=15445.msg148808#msg148808 (http://el34world.com/Forum/index.php?topic=15445.msg148808#msg148808)
Look at how/where SG and DL put the tube sockets with respect to the barrier strips. This is very important. Their way they don't need shielded cable for grid hook up runs. Unless it goes to a volume/master volume pot that's far from the tube.
Keep the PI tube close to the output tubes it feeds.
Picture link;
http://el34world.com/Forum/index.php?action=dlattach;topic=15445.0;attach=37379 (http://el34world.com/Forum/index.php?action=dlattach;topic=15445.0;attach=37379)
Brad :icon_biggrin:
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Thanks brad. That's a good point. I am going to arrange it today. What I will do is build my board up on blocks and out the tubes on the bottom side so the pins are face up on the flat of the chassis. I will move them closer to the terminal strips. Those breadboards are great. I have seen those. But I wanted mine to closely resemble the final chassis as possible. This way my wire routing and everything will be similar from bread board to build. I will also store my transformers underneath as well and drill some holes for the center taps to come through and hit the ground lug as well as holes for my heaters and power.
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But I wanted mine to closely resemble the final chassis as possible. This way my wire routing and everything will be similar from bread board to build.
I don't see that working in a practical way. I don't see how anyone could do that. Maybe on a small single ended build like a Champ not anything else. As the circuit gets bigger it will be harder and harder to make that happen. Doesn't look like it now with the amp circuit you have in there now, to me.
It's your bread board and you can build it like you want but I wouldn't do what your describing. Think about this;
1. It will be hard to change tubes.
2. You won't be able to see if the power tubes are red plating.
3. It will be harder to change PT's and OT's for new/different builds.
4. It will be a major pain flipping that board over to get at things underneath and then flipping it back again.
(Also I'd be afraid to use that metal sheet under all the barrier strips and around the tube sockets. If a connection with B+ on it comes loose and touches that sheet now the whole sheet is hot. I'd take that sheet out.)
The way that DL and SG made their bread boards have been proven by them to work very well.
Brad :icon_biggrin:
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Do those guys have metal under all their strips?
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Do those guys have metal under all their strips?
Why?
I don't think so. Why do you think you need them?
Brad :dontknow:
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I dont think I NEED it. I just assumed it would be convenient. And judged mine by a couple of the photos in that thread. Whatever a couple of them have riveted down underneath their components looked like thin sheet metal to me. Either way, a new breadboard will be something I will have to research and build very soon. But for the time being I am starting from scratch. I admit and have learned that my breadboard was not convenient, however at the moment it is what I have to use for this build. I made some modifications to it today. I cut the back lip off where the tubes were mounted, I mounted tubes on bottom side (so I will just be soldering all my wires on the sockets. when I have money to put towards a breadboard I will by the sockets like the other guys are using) I removed the metal underneath my strips for a little added safety. I know that theres a lot of metal still but I did remove a lot of it. Anyway...this is not the thread for breadboard building but this is where I am and I will start over on my build and make it much cleaner. Will report back soon. Thanks for your help everyone.
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I honestly didnt think I would have so much trouble with the build on the breadboard so I just threw it together. I guess thats probably where a lot of my trouble came from. Welp....here I am starting over. I should have known better than to get in a hurry and slop it together. Anyway...Sluckey...I am embarrassed. Being an electrician I should never wire anything that sloppy even if only temporary. I guess I learn the hard way. So far its all pretty neat. Let me know what you think of layout so far. I have the filter caps all grounded at the transformer lug rather than on busbar like before.
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If you're a carpenter the doors stick in your house, a plumber, toilets need work, electricians, faceplates missing. I think it's a prerequisite to being very good at work!
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Amen Shooter
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That's looking much better.
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What are to 2 mini switches by the filter cap and trany for?
Brad :dontknow:
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Ones the power and the other is standby
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I wonder if you're about to tell me to check my ratings on the switch. I didn't think of that. Oops.
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I'm not sure those switches will be able to handle the power, voltage and current.
Be safer to use a larger body switch with higher voltage/current handling.
Brad
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I wonder if you're about to tell me to check my ratings on the switch. I didn't think of that. Oops.
Yep. :icon_biggrin:
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I'm really glad you caught me on that. No it won't handle it. Not even close actually. Ha. I'll have to swap that thing out.
35vdc :BangHead:
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Do you really need a standby switch on a breadboard?
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A Carling SPST switch like Fender used for decades, for on/off and standby switch, is rated at 6A/125v, 3A/250v.
They get away with using a switch under rated for voltage because their not drawing, more than what, 400mA's at full tilt in a 4x6L6GC 80w Twin?
Doug's got them and other switch's that will work.
Brad :icon_biggrin:
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Thanks Brad. I'll check into those.
I guess not Sluckey. The standby on this build is optional anyway.
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Thanks Brad. I'll check into those.
Here's a link to Doug's store, switches section.
http://hoffmanamps.com/MyStore/catalog/parts7.htm (http://hoffmanamps.com/MyStore/catalog/parts7.htm)
And you can find it at the very bottom of the page.
Brad :icon_biggrin:
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Got her all wired. Power amp and one full triode preamp (with pre volume in between stages. I want to add a PPIMV so I can get an idea what this preamp will be like without having to have it full blast. Can you check this and tell me if its ok. I dont think I have to add anything since its a cathode bias but I may be wrong :dontknow:
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I would put the cut control before the MV.
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will do. Will report back tomorrow :icon_biggrin:
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After a lot more patience and way more hours of work than I should have spent in a few days, the amp is up and working. There are a few things that I noticed that I would like to tweak. The preamp volume knob is too drastic. It's a 1m, which is what's on the 2204 but I've been told 500k is standard.
The tone stack doesn't seem to affect the frequencies as much as I think it should. The bass seems to. But the mid and treb knobs have little affect on the tone. It is a 2204 tone stack.
It is also a tad gainy and muddy. I know the preamp values need to be adjusted. I assumed the muddiness would be a result of cathode cap value being too high but the only cathode cap I have is a 4.7uf. I did goof and placed it on the second half of the first triode rather than the first. So the first half of triode only has a 1.5k. Not sure what affect this has but I will play with that.
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Been playing around with the amp over the weekend. I got all the bugs worked out except one. I was hoping someone could help me out.
I installed a PPIMV. It seems to function fine except when the amp is comepletely cranked (all the way up and maybe just a tad shy from being all the way up) I get some wild oscillation like a consistent thum-thum-thum-thum. Im using a dual 500k. I also installed the cut control before the PPIMV. Im guessing it has something to do with the PPIMV not being wired with shielded cable??
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Im guessing it has something to do with the PPIMV not being wired with shielded cable??
Could be, got some shielded cable left?
Brad :icon_biggrin:
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I'm all out but I'm ordering some from Doug tomorrow. We will see how that goes. I'm guessing it will probably take care of it. I keep double checking my PPIMV but there's just not a lot to it especially on cathode bias.
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Sluckey,
You and i discussed my OT a few times. i was just wondering if you can tell me what kind of differences I could expect if I were to change out to an OT designed for this circuit. Like I mentioned before, right now this is for a 30 watt marshall. I have what I feel is a bit of woofieness in the low end. It can be dialed out but the low end is pretty dominant even with the bass being set somewhat low. I really don't think its in my preamp design as I don't have any low pass filters and the only cathode bypass cap I have is a .68 on the first triode. I actually have multiple hi pass filters. Just curious what you thought.
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Enjoyed scanning through this thread and seeing your approach!
I suspect that the high valued cathode bypass and coupling caps are contributing to the woofiness that you describe.
All of these changes will help to dial out the high bass content that I think you want to reduce. These are based on the schematic presented early on in the post. Please forgive me if I didn't catch any changes to these values that have been made and possibly overlooked at this end.
C1a to 0.1 uf
C1b to 0.1 uf
C2 to 0.022 uf
C5 to 0.1 uf
C9 to 0.022 uf
If that gets too shrill, increase them halfway between the suggested and current values. Or...start by changing to values between what you are at and the suggested reduced values. Try doing them one at a time so that you can calibrate your ear to the difference that each one makes.
Rock on,
Dave O.
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ooops....forgot to also mention C14 (power tube cathode bypass cap). I haven't built many cathode biased output tube amps but will mention that the same philosophy applies there as well. Lower values will decrease the low frequency content of the signal. I am not sure if halving C14 is advisable due to biasing needs...but....I am sure that reducing it by 25% will make a noticeable difference.
Hopefully someone else with more experience will confirm that feeling.
Good luck!
Dave O.
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Thanks a lot for the response Dave. Actually thats my fault. I should have menioned that the preamp schematic posted earlier no longer applies. I have since changed all coupling caps to .022 and I only have one cathode bypass cap in the preamp. The first triode consists of a .68uf bypass cap. I am in process of redrawing schematic. If I dont get much activity on here I will post it in a day or so.
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No sweat mate! It is nearly impossible to capture in print every little change made to a prototype amp.. Trust me....I know...and I seem to keep making the same "mistakes" over and over again. Fortunately....sometimes they are happy "mistakes"!! LOL
I saw mention of the 2204 preamp earlier in the thread. Is that where you are at right now?
Cheers,
Dave O.
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No sweat mate! It is nearly impossible to capture in print every little change made to a prototype amp.. Trust me....I know...and I seem to keep making the same "mistakes" over and over again. Fortunately....sometimes they are happy "mistakes"!! LOL
I saw mention of the 2204 preamp earlier in the thread. Is that where you are at right now?
Cheers,
Dave O.
The tonestack setup is at least. HA. The first preamp tube is probably close with that .68uf cathode cap. But all other parts of preamp were tuned to taste. I had to do a lot of gain reduction since the original preamp design. I pretty much have 100k plate resistors on the entire preamp and I think the smallest cathode resistor I have is a 1.5k. Also have a 10k on cathode resistor and the last cathode i believe is about a 4.7k. I have a standard high pass filter on preamp volume. And another on V2a. Also opened up a couple grid resistors a tad.