Looking for Bogen CHB 20A Schematic (Bogen CHB20A Project)

[Platefire]

I thought I remember seeing schematics on this site for a lot of Bogan CHB series amps but I just looked and didn't turn up nothing. Anyway I was given a Bogen CHB 20A amp today and was wanting to see a schemtic to see what my options were in converting it to something. With only one preamp tube and two power tubes, it's got me puzzeled on how the amp operated on so few tubes. Platefire

[Platefire]

I'm Sorry, I jumped the gun! I found it under PA Amps and see that the ECL86 is a dual preamp and power tube--thus the shortage of tubes in this amp. Should be interesting! Platefire

[sluckey]

Look at Geezer's Little Wing.

[DummyLoad]

I'm Sorry, I jumped the gun! I found it under PA Amps and see that the ECL86 is a dual preamp and power tube--thus the shortage of tubes in this amp. Should be interesting! Platefire

6gw8 - nice! i posted an amp built using those and a 12BZ7 - called the goliad - it's very similar to the hoffman 18W stout.  definitely a cool tube.

--DL

[Platefire]

Har! Like old times agian. Taking an old junked amp and trying to bring it back to life again. I first hooked it up to a test speaker and plugged my guitar in aux input---nothing through the speaker! I took the tubes out and checked voltages and it's getting good heater and DC voltage all accross the circuit. I cleaned the tube pins, sockets and pots with contact cleaner, put the tubes back in and now getting sound through the speaker--so I got a working amp--well the PT and OT is working anyway. So now that I know I got some working major components, now I can plan an amp Boy I love it!

[Platefire]

I got other things I need to be doing but for some reason I been foolen with this amp. This morning I cut out the aux input wiring, removed the aux input vol pot, moved the mic vol pot/with wiring over to the right in the aux pot hole and installed an input jack in the mic pot hole. I cut the sheilded mic input wire to the threaded jack in the back and wired it to the new 1/4" jack. I decieded I just wanted to hear the existing circuit as is. I hooked it up to a 1-10 cab and fired it up. I've always heard how some players liked the existing Bogen circuit as is but I never gave them a chance and always gutted them to  build something new.

You know I'm kinda impressed with what I'm hearing. The thing is dead quiet even with the old circuitry.
The tone stack even sounds like it's doing a pretty decent job---but I've been testing with a tele---the tru tone stack test will be with a strat or les paul. So I'm pleasently surprised that it is sounding so good with so little work done--it has to be to good to be true. Better go do some real work done before the wife begins to get on my case.   besides I'm suppose to be working on my TE-Deluxe!

[tubenit]

IF you do a major conversion ........ here is one to consider.  The 6GW8 and 6BM8 have some similarities. You could eliminate the PPIMV if you wanted since it would not be a loud amp to begin with.  And you could use a fixed value for the mid pot if needed.

with respect, Tubenit

[kagliostro]

Schematic for a possible conversion here

http://photobucket.com/images/Bogen+CHB20A+Schematic/

Kagliostro

[Platefire]

Thanks Tubnit and Kagliostro

I saved the schematics on my computer. I had already downloaded the LWR by DeGeezer which I know tubnit worked on too. The 56T looks highly related to the LWR. I've also been interested in a Marshall 18 Watter for a long time. I appreciate your help and looking at what options I have on this thing.

Right now I'm still recovering from the shock of my first impression of the stock Bogen circuit. I reworked a few PA heads since I started working on amps and in every case so for I've never liked the
stock tone stack and response but this one is appealing! I will have to work my way through this one before I can move along with any serious modding. I have been studying the Bogen schematic and it's hard for me to tell what's going on in the power section---I guess it's PP but hard for me to understand.  If you can tell me what's going on with that Bogen power section, I would appreciate any help.

I am considering some slight changes to the stock circuit and would like your input of what you think. I have a poorly marked up scan of the schematic if you can read my Hen scractching. What I'm thinking about is rewiring to move the location of the tone stack between the two preamp gain stages and moving the MV to right before the PI that I try to show on the mark up. Let me know what you think? Platefire

[sluckey]

The power amp is a classic fixed bias push/pull amp driven by a paraphase inverter. There's a global NFB loop. The power supply uses a voltage doubler for the B+.

[Platefire]

Thanks Sluckey

Can you explain global NFB Loop?

[sluckey]

Local NFB is negative feedback within a single gain stage and only affects the gain of that single stage. Global NFB usually encompasses several gain stages. In this case, the negative feedback source is the OT secondary. The feedback signal wraps (loops) all the way around to the first half of the paraphase inverter, a more global circuit. The global negative feedback circuit reduces the gain of the PI and the PA, tightens up the sound, improves fidelity, and decreases distortion.

[Platefire]

I've been very familiar with local NFB for a long time but this is my first encounter with global that I'm aware of . That expains a lot of the extra wiring I couldn't understand. Platefire

[Platefire]

Got more questions.

1-In the preamp stages for the Aux and mic channel there is a 470K Resistor in parallel with a 100(pf?) cap. Is this a high pass filter to increase highs and reduce gain or what is the purpose? I did away with all this on the Aux input including that cap and resistor  because I didn't need that imput and did away with it to make room for the 1/4" guitar(mic) input on front panel. So I guess what I'm getting at, if I wire the tone stack the location where the parallel res/cap is at in the mic channel, do I need to keep this or do away with it or keep it inplace to retain the existing integrity of the amp.

2-Also If I re-wire the Master Volume right before the phase inverter--I'm thinking I need to do away with the 100pf cap to ground and seperate 1 meg R to ground. I guess what I'm wanting to do is retain the existing intergrity of amp but move the master volume to where it will be more effective to work as a conventional MV operation. I guess bottom line is I can rewire--and if I don't like it put it back like it was??

I know most of you don't agree with the wisdom of preserving the integrity of an old PA head original circuit but I tell you there is something here that I'm really liking and I don't want to destroy it by making quick surgical extractions without knowing why and what for. Platefire

[Platefire]

Gave it a pretty good road test with my band and she held up pretty good. Power range about like a tweed deluxe. Had it running though my home made 1-10 cab with a 20 watt Eminence Alinco Legend. Here is a picture of the rusty crusty thing. The circuit operates just dead quiet even with all the original circuitry and original bogen tubes. It takes my pedals very well. It's really responsive if you crank the channel volume to 10 and adjust your overall volume using the Master Volume. I think it looks and sounds cool as is. I think I will wire in a standby switch and 1/4" speaker out on the 8 Ohm tap and call it good for now. I can always change my mind and mod it later. Platefire

[gldtp99]

Bogen CHB20A's ? I still have about a half dozen of them laying around------ modded/sold some on ebay---some locally----haven't messed with them for a while now.
Here's a poorly done vid demo of a CHB20A (scroll down to see vid)---- done several yrs ago by some friends with a (then new) digital camera that would take short vids---- the camera's mic is terrible (sounds like amp is oscillating, but it's the camera mic)---- pretty loud for the camera mic thru the white Sound City 4x12 w/G12H30's.
This is a modded Bogen circuit, not a gut-and-rebuild to another circuit-----i liked just brightening up the existing Bogen circuit on these CHB20A's----adding adjustable bias, new filter caps, etc, as needed.
Again this is from several yrs ago---- i'd probably do something different today on a CHB20A re-furb....................gldtp99

Link: http://www.myspace.com/gldtp99

[Platefire]

Thanks gldtp99

I live out in the country/rural area and am on dial up! so it takes forever to download any type of large file--and I just don't have the patents to wait. I did click on the link and it ran a long time and never did open up--but thanks anyway. The only option I got for faster net is Hughes net and I hesitate to pay that on my retirement pay  

I'm still scratching my head on this bogen cause it sounding pretty dog gone good to my ears and basicly all I've done is wire in a 1/4" input directly to the mic channel using the same old existing shielded cable that ran to the threaded mic input on the back. I ran in through a 1-12 cab tonight with a 100watt Carvin British Series speaker and it sounded a whole lot better than the first 1-10 cab I ran it through. Besides sounding good it also responsive enough to where its allowing me to play pretty good to the point of inspiring me to do a little extra than normal and that's making me pretty happy with it.
I just can't figure why I'm the only one thats liking it as it is??? It seems to have plenty of treble/highs for me. Maybe I'll get over it, I don't know? but right now I'm having to much enjoyment out of it to change it---so I'll just ride it out and see where it goes.

My CHB-20A is missing the folding back metal panel. If you got an extra one of those, I would like to strike up ad deal with you on it. Platefire

[gldtp99]

I was playing humbuckers exclusively back when i was messing with mine----- the CHB20A's would tend to flub out on the bass when played at, or near, max (only way i played them then)---- so i really limited some bass rather than brightened them up---- changed to some smaller value coupling caps, as i remember.
I liked the stock circuit, other than that-----these amps often would come with a Mullard 6EU7 and Mullard or Telefunken ECL86's.
Bogen also made other small PA heads, of the same era, with ECL86 push/pull outputs--- i have a CHB14 (blown PT) and an MU10-- both have tube rectifiers so they don't have the voltage doubler PT of the CHB20A and both have cathode biased ECL86's vs. the fixed bias CHB20A---i keep the MU10 at my shop to show people how a 10 watt head can sound thru a 4x12 cab--- this one has no tone stack, just Bass and Treble cut slide switches and it sounds like a cranked Marshall JCM 800 when dimed---just at a 10 watt vol level.
I'll look for a flip-up back panel but i think i threw them all out when moving my shop---i'd remove them so the amps were easy to carry one handed---i remember throwing an armfull of them in the dumpster a few yrs ago---i still have many CHB100, CHB50, CHB35, CHB20, etc, etc in storage--- if i find one you can have it.
I'm currently re-building a Challenger (by Bogen) CH30 to Fender 5D5 Pro-based specs----except with a cascased preamp and a "balance pot" for the paraphase PI---i gutted the orig circuit and will re-use the orig OT--- orig PT not usable--- new PT from Magnetic Components---- 6L6GC x 2, cathode biased, 5U4GB Rec, three 12ax7's for pre and PI----should be a cool sounding amp.......................gldtp99

[Platefire]

OK, Thanks! Of course the amp is fine without the backplate. I become kind of fond of the amp since I been jamming on it and have since added rubber feet(that was missing) and also a fender type handle out of extra parts I had. Usually I want to paint these old amps and make them pretty but I like this one in is old rusty crusty state--I think looks cool as is. So I understand about cleaning out old stuff and have done that serveral times myself--you tend to accumulate a lot when you been in this a while. So if you turn up a backplate I would be happy to send you shipping and a few bucks for the plate.

I'm pretty familiar with the CHB line. I've have a CHB-50(thats kind of rare) that I converted to a 5F6A.
The OT was bad and I put one of Dougs bassman type OT in there and man-what an improvement. It sounds and works good but I did it in my early amping days and it's all old style point to point without a board(a lot of terminal strips)and my grounding habits/experiance were non-existant--so the amp has a little ground hum which really annoys me because I like a quiet amp. So I've been planning to go back in there one of these days and see what I can do with it but haven't yet. I also had a CHB10A that I completely gutted, rebuilt with a princeton type preamp/tonestack that was amazingly loud for what it was. I kept it a few years and sold it on e-bay. I tried the CHB20A yesterday with my Les Paul and just cut the bass to about 9:00 and boosted the treble to about 3:00 and jammed on it a while and was happy with it. This was in my living room and the real test would be playing out somewhere with it because at louder volumes really tells the tale. Platefire

[Platefire]

On this amp, I was wondering about trying relocating the MV after the tone stack prior to the PI but I find the MV pretty useful even in it's present location. Even though in it's present location doesn't have the dramatic effect of normal guiter amp MV operation, it is useful to a certain degree.

I was wondering if you could grab the grid signal just before and after the existing MV location and run it to a DPDT switch, also interupt the grid just before the PI and wire the switch to switch between MV locations? Seems workable but I may be missing something! Platefire 

[66merc]

Howdy folks, just running through some old posts.

In the preamp stages for the Aux and mic channel there is a 470K Resistor in parallel with a 100(pf?) cap. Is this a high pass filter to increase highs and reduce gain or what is the purpose?

Never caught an answer to this.  I was wondering this myself as I am wanting to mess with one of the stock CHB20a's I have lying around.  What would be the result of removing this be?  BTW I have already canned the aux input and extra volume pot.

Also, I'm wanting to reduce gain.  Normally the easy fix for me (being a perpetual novice) is to switch 12ax for 12ay or other low gain 12a? type.  Darn Bogen and their 6eu7's!  I really don't want to rewire for 12a? since I have a box full of 6eu7.  Would adding a cathode resistor/bypass cap on the first preamp stage help?

These little CHB20a's are fun amps.  I also have a screamer that Gldtp modded a few years back.  But this next one I want to tame down a bit to get more clean headroom.  Luckily I've collected a few sets of these tubes over the years since the 6GW8 is getting somewhat expensive.

Thanks

Eric

[Platefire]

Eric

I know what constitutes a good tube amp is way subjective according to the individual. I'm mostly a clean player that uses the guitar volume like the old blues players and like it right on the edge of breakup with my guitar volume fully cranked. I use my Route 66 pedal for pure distortion. I'm really finding the master volume handy on this stock amp even though it's in a location not normal for guitar amps. For my Tele or Strat I can crank the pre-volume 100%(10), adjust my overall amp volume on the master vol about 1:00 and get my tone and feel. With my Les Paul that has pretty hot 12K Ohm humbuckers with Vol#1 about 8:45 and Vol#2 at 1:00 I get about the same response. So the volumes are actually usable in my case for cleaner sounds and good compressed start of breakup at top volume. Of course this amp will get into big time breakup if you keep cranking the volumes up.

I finding the tone stack so far very functunal for my guitars. I like kind of a fat tone with just enough highs to break through on a leads but thick enough to get a good chunky rhythem sound for just backup rhythem sound. On my tele/strat I'm finding that set on "0" for bass and treble--not boost or no cut. On my LP and humbuckers---cut bass -1.5 and boost treble +2. So you see I still have a lot of play on the dials for more control.

The 6EU6 is sounding oK to me. Even with 58VDC(shows 68 on the schematic) on the first gain stage. Second 6Eu6 gain stage plate is 147. I'm getting nice compressed feel but still clean even when the guitar volume is low. All other voltages on the amp are real close to the schematic listings. Power tube plates are running 395 and are shown 400 on schematic. Screens 374/375 Schematic.

I'm still in the honymoon test out period with this amp but I continue to be impressed with the stock PA circuit. Platefire

[PRR]

> a 470K Resistor in parallel with a 100(pf?) cap

Got wire-snips? Clip the cap and listen.

The resistor is part of the mixing system (so sources don't fight each other). There's stray capacitance everywhere. When audio comes through a huge resistor, stray C sucks the highs off. The 100p _appears_ to attempt to correct this. A moment with a calculator says the cap in 100pFd||470K works only on the top octave of useful guitar harmonics (above 3KHz), so may be moot for your use, though important to pass a 50Hz-15KHz Acceptance Test for a contract sale.

[Platefire]

OH NO! The dreaded stock Bogen is back 

I said I would shut up after the last statement--but putting this thing on a nice efficent speaker makes as much difference as daylight and dark. The honeymoon is aging and it's still sounding good. I'll really try to shut up now. Platefire

[Platefire]

Hello

I'm still happy with the stock Bogen CHB 20, I just would like to relocate the MV to pre-Phase inverter to get higher gain at lower volume---or because tone stack is behind gain stages--would this change not accomplish my purpose?

Schematic is attached. It will just take rewiring the existing R5 1 meg master into that location. I'm just wondering with the MV removed from it's current location, do I need to rewire R13 (1 meg grid to ground) in that location and let R5 take R13's location. I would like to make this change without negatively effecting the existing sound, just better gain control. So if this change won't accomplish this, I would appreciate your input. Thanks, Platefire

[Platefire]

Well! since none of ya'll came to my rescue, I sucked in my insecurity, prayed and went ahead with the MV relocate. First try after re-wire the MV wasn't working with bad hum. Found the gound on the MV wasn't working and ran a new ground wire---that did it--nice a quiet now.

The master volume sure did what I was hoping for and gave the me more control over gain and overall playing volume. I can now jam at a low volume clean punchy tone with great response. Seems like the original integrity of the amp is maintained with the exceptions below.

On the negative side with the pre fully cranked and MV low the distortion is way flubby for my taste. Even though I wouldn't use this setting hardly any--be nice if it was tighter. Also the overall gain increased to a point where I think I lost some of my overall loudest clean headroom. May need to tweak with a dropping resistor---still open to suggestions on that!

So basicly what I did was not change the existing circuit it any way other that un-wiring the MV in its existing location, connected the grid wire direct where it was removed and wired the tone stacks trebel output grid wire to phase inverter through the MV putting it just before C10 and R13 on the schematic. Platefire

[DummyLoad]

cool! glad you got it working.

peace.

--DL

[Platefire]

I haven't had much time for amping lately. Sure felt good to melt some solder and move a few wires around again. I've been jamming on it off and on all day and pretty happy with the results. Looks like before it's over I will end up putting a MV on everything I got. I've got a Bogen CHB-50 and Dukane PA heads with Bassman circuits and no more room/space for another volume control--I will have to use a ganged pot for those. Platefire


BTW-On the Pignose G-40 they modified the Bassman circuit slightly to better work with a master volume. I'm wondering if I should use that circuit as an example for adding a MV or has anyone had some experiance at adding MV to a 5F6A?

[HeyItsBen!]

Hi all,
I'm trying to play along at home, and I think I'm close to 'schematic-ing' what Platefire is doing with the Master volume.
If anyone is still reading this old thread, could you take a look at this and confirm/correct?
It's my first ExpressSCH attempt- I expect mistakes...

-Ben

[HeyItsBen!]

Annnndd, first post noob mistake!
Here's the much-smaller-file-size image I meant to send with that last post, dangit.

[Platefire]

Believe it of not I'm back on this thing again. I fired this up the other day and it just seemed to distorted if you put any volume on it past 2. So I rewired the volumes & tone stack more or less like "HeyitsBen" in the previous post schematic was drawn up ---still to distorted.

I decided to start reworking it from the input on out until I get some good results. Tonight I de-soldered the V1 wiring to the old 6EU7 socket and trashed it all. Going to rewire the existing socket for a 12AX7 so I will be able to use all the 12A____variations. So I will be rewiring V1 to be guitar friendly in a stock fender input kind of way. The plate voltage on V1A is 59V with a 470K plate resistor and V1B is 150V with a 220K plate resistor. I was thinking about going with 100K plate resistors and getting the plate voltage up around 250V that would hopefully help the clean headroom. Gosh!!! those old Bogen workers made some tough mechanical connections for their solder joints---it was a dickens of a time getting all that loose on the old socket without destroying it

I was also thinking of going to a more guitar friendly tone stack. I just need to use the two existing 1 meg pots(treble & Bass) with the long split shaft to work with my existing Bogen Knobs and Cover. I was thinking the standard two knob Princeton/Harvard/Deluxe but they all use two 250KA for bass and treble? What effect to the sound would using the existing 1 megs with that type tone stack have as opposed to the fender 250K ones??

Guess I just had the desire to tinker with an amp some--so if I could get a bit more clean headroom and use the MV to get a little grit also would be nice. Platefire   

[DummyLoad]

aside from a dying tube, perhaps it's a leaking coupling cap or a shorting bypass cap. check the output stage and preamp bypass caps. to test the coupling caps, lift out the grid end of the coupling cap and measure DC: if you see over a few millivolts, replace it.


check B+ taps, if any or all are abnormal (low), check the power supply caps.


--pete

[Platefire]

Hay pete thanks! I did double check the voltages in the output amp section. One abnormally I found is on the schematic it shows the plate voltage going to the PI V2A 180V but it is reading 275. The power node feeding the PI's reads 280V correctly. Next is the 220K resistors plate resistors. Other PI plate on V3A is reading correctly at 130V like the schematic. The 220K resistor on V2A is only knocking the voltage down 5 volts (280 to 275).
I did measure the ohms in place on both those resistors and they did read close to 220 Ohms. I don't know if it means anything, but one power tube is running hot and the other cool to the touch?

So I went and switched the tubes. Now both PI's Plate voltage at V2A and V3A are now 130V. Both tubes are hot to the touch now---but according to the schematic V2A should be 180V!!

So I got three 6GW8's, one NOS and the two that came in the Bogen. The NOS is in one of the sockets already. So I swapped the used one in there for the other used one. OK I measured voltage again and now V2A is 171v and V3A is 130V plate voltage. Both are hot to the touch. That is better aligned with the schematic. One of the tubes must have been weak.

One more thing that may be an issue. Bias voltage on schematic is -16 but is reading -12. May need to replace those bias caps.

That may have been my problem all along. I can't test the audio until I rewire for a 12AX7. So I may get into that tomorrow. Thanks, Platefire 

pdf schematic is attached. 

[Paul1453]

Would one of you guys happen to have the Bogen K10 schematic to post here?

[Platefire]

You mean like this one?

[DummyLoad]

One more thing that may be an issue. Bias voltage on schematic is -16 but is reading -12. May need to replace those bias caps.

absolutely: always replace those. just because of age and regardless if they are still working.

with -12V bias check for red-plating with the lights out. the -12V bias may have contributed to premature demise of the cold tube. replace C15 (.22uF), C16& C17 (25uF) electrolytics. note polarity. ;-)   

--pete

[DummyLoad]

You mean like this one?

that looks familiar!   


hacked mine into the attached schematic. kept the paraphase. ditched the tone selector and used "5E3" tone control, and modified 6AU6 bias - originally it was grid-leak bias: it was changed to cathode bias. eventually added a "presence", however can't locate the schematic with what i did.


--pete

[Paul1453]

You guys are the best.   

[DummyLoad]

here's the original k-10 thread tread i started:  http://el34world.com/Forum/index.php?topic=7404.msg167532#msg167532


let's not clutter up plate's CHB20A thread... 


--pete

[sluckey]

Platefire, here's a preamp that will work very well with that amp. This particular schematic is for an Ampeg Gemini II, but this identical preamp appears in many other Ampeg models from that era. I've just added this preamp to a Hammond organ amp. Sounds very guitarish.

You may notice that it's almost identical (except for component values) to the preamp you just ripped out. 

EDIT... added schematic with voltages.

[Platefire]

Thanks sluckey! I looked through a ton of schematics last night looking for something that would work with what I got and didn't turn up anything. I even looked through Doug's Ampeg schematics but guess I missed that one. So that looks good to me. The only thing I was wondering about after looking at that Ampeg preamp/tone stack is the MV. I may not really need one because I'm mostly interested in clean, but if I was to use the extra 1 meg vol I got, I guess it would go right after the 270k resistor? Platefire

BTW--I haven't ripped the existing tone stack out yet , only all wiring related to the 6EU7 to rewire for a 12a---types. Also, do you have any plate voltages for that preamp I could kinda aim at?My "D" node voltage is going to be close to 280V.

[sluckey]

New pic showing voltages and MV. This circuit should work fine with 280V but if you want higher just add an extra resistor filter cap node. Connect it to that 375V node on the schematic. Change the resistor value for whatever supply voltage you want. Ampeg actually used 280V supply on the Gemini II.

[Platefire]

Thanks Again! I just need to go through my existing inventory and see if I can come up with the different parts.

I think I will go with the existing 280V first to see how it works. I think I discovered one of my main problems last night with a weak or bad power tube and after swapping the tubes around, think I've got a good working pair now. It might have sounded pretty decent if I would have discovered that first but I think the 12AX7 rewire
will make it a lot more maintenance friendly for myself and any future users.

Thanks for the nice schematic! I think other Bogen CHB-20A hackers like me may be interested in that also. Platefire

[shooter]

question on the schematic sluckey posted, the TS shows a 120k/270k between the bass and treble wipers.  What's their job?  I'm drawing up an amp that uses essentially the same type TS, without those R's

 

[sluckey]

The 120K between the bass and treble wipers... I've always seen a resistor in that position on james tone stacks. I just always considered it to provide some isolation between the two pots to cut down on interaction? Just a guess.

As for the 270K from bass wiper to ground... I don't see that very often, but it is on the Ampeg Gemini II and several of the B15 amps. But you won't find it on the Gemini I or several other Ampegs either.  So, I guess it's not really necessary. Without that resistor, the following grid stage has to rely on the Bass wiper to provide a ground return path. Maybe Ampeg put the 270K there to provide a dedicated ground return in the event that the Bass wiper fails? Again, just a guess.

[shooter]

just a guess

I'll accept your guess, thanks sluckey

[Platefire]

sluckey

I'm looking at parts to accomplish the preamp on the schematic you provided. I'm confused about V1B cathode bypass .1 cap. I can't find an electrolytic cap with .1 value. Is that suppose to be a regular coupling type cap?
Platefire

[sluckey]

I can't find an electrolytic cap with .1 value.

Me neither! 

[PRR]

> What's their job?

The B and T on a James always interact. This helps reduce that.

The value makes very little difference. For impedance range as in Sluckey's diagram, 100K to 330K just audible. Higher values tend to shift turnover point to lower frequency.

270K from bass wiper to ground gives less bass at "5" (or "0" hi-fi knob scale). This may be to shim the "flat" setting when you can't get 10%-taper pots (many pots today are 20% taper). Or maybe to voice the amp. I'd think you could omit it and just set the bass knob to-taste.

[Platefire]

No .1 electrolytic to be had also the bias has a .22/400 cap coming off the PT Red secondary can't find no such animal either are these caps of the past, no longer available? Platefire