oh lordy... another bogen CH30

[DummyLoad]

somebody make him stop...  my local tube supplier called and said he ran across something that i'd probably like. i make the pilgrimage to his shop since i need some 7868 anyhow, and lo and behold, he's got another bogen ch30 - in better shape than the last and the OT isn't fried. 35 clams later, i'm the proud owner of another purple and silver and rust colored PA amp. OK i know how to do these... but where to begin, leave/mod the same tube line-up? no, been there, done that... hummm...something all octal. duh! 

something i've been playing with on the bread board - an TW express like all octal layout. here's what i decided to do; input > 6SL7 gain > switchable f-3dB & 1/2 james (treble) > 6SL7 gain > 6C5 gain > 6SL7 paraphase PI & cross-line MV > 6L6GB P-P self-biased.

if i had to do over again, i'd scale the f-3b cap stack starting at 500p - ending in 4.7n. alas it's such a pain in the never mind where to wire up and even more painful to re-work, i just left it alone.

interesting tones. overdrives really nice and delivers well with the clean tones with good dynamics, just roll the MV and gain back a bit. 6J5 is definitely a valve i'll consider in the future.

schematic attached - i'll post pics later today.

--DL

[mresistor]

Cool amp. Looks similar to what I've been playing with..   

[rzenc]

Nice!

I really like paraphase PI..

DL, ins't 6J5 a close relative to 12AU7?

Best.

R.

[thelonious]

Cool! I'm in the middle of a similar octal build with a Stromberg-Carlson PA amp. I've never played with the switchable f-3dB cap stack before - do you have a source you like for that 4-way rotary switch? I saw that Doug has a 3-way in stock for impedance selection, so I could always use something like that instead I guess.

Cheers!

T

[DummyLoad]

thanks guys!~ 

the switch came with the amp - i salvaged it. took some coaxing to get the shaft to turn freely again. a couple of drops of sewing machine oil and working the switch did the trick! 4TSP switch can be had at antique electronic, mouser, etc. don't see one in doug's catalogs either.

6C5 is so close to 12AU7 we don't about the variance and use the same tube model. 

i installed 5691s simply to torment audiophools. those things are flipping on fleabay for about 100 bux each or more. damn! i wish i had a case of 'em. was thinking of installing a 5931 recto and a pair of 5932 tubes for that industrial motif.

--DL

[Ed_Chambley]

You are the PA hack man.  The hack you gave me for the Stromburg was amazing.  What a cool old sounding amp.  I got an interleaved OT for it, so I am going to switch it back as I came across a late 50's wooden turntable and a Bell and Howell movie theater speaker.  The field coil speaker is trash, but the housing is cathedral style.  I've got a ribbon tweeter and  a full range 12" and more crossovers than you can shake a stick at that will fit it perfectly.

My question is if I make the transformation to gain the hz range, will I lose the ability to use it for guitar as well, using and extension cabinet of course or would it be better to have both transformers to switch placed on a stitch so the original OT has the guitar signal and the and the interleaved tranny is applied to the circuit when switched to phono/aux.

The cap and resistors are no problem as I have already got a way to wire for audio and guitar as well.

[Tyrannocaster]

Hey, that IS nifty! I'd like to try that only I don't have any octal preamp tubes. You mentioned the 6C5/12AU7 sub but what about the others? I have heard of the 6SL7 but basically, unless it is a power tube I know nothing about octal tubes. Those old preamp tubes are a mystery to me. I do have a lot of AX7, AT7, AY7 and AU7s, though. Will any of those work without major re-jiggering of the circuit?

[thelonious]

From what I understand, a 12AX7 generally can sub for a 6SL7 in terms of values and voltages. The main differences are 1) the base--of course--and 2) the bass. Octals sound "fatter", but sometimes have more problems with microphonics and/or heater noise. Not sure why either of those is the case, but that's what I experienced in my octal build.

[Platefire]

U Da PA Man! My scores are a lot fewer and far in between 

Last one handed to me was a Bogen CHB20A but I'm still amazed that I only did a little re-arranging and got a great working amp. First one I ever got that I didn't have to completely rework to get my sound. Plate

[DummyLoad]

6SN7GT - similar to 12AU7 - 6FQ7/6CG7/12BH7 is the same thing in a 9 pin - \
5692 is a high reliability/long life/low microphonic version of the 6SN7 - they are prized by audiophiles
6SL7GT - closer to 5751 than a 12AX7
5691 is a high reliability/long-life/low microphonic version of the 6SL7 - they are also prized by audiophiles
6F5 - the granddaddy of the 12AX7 - has a grid cap
6SF5 - the daddy of the 12AX7 metal or glass package - glass is not as common
6SC7 - like a 6SL7 - only has the cathodes strapped together internally and brought out to the base on a single pin. metal or glass package - glass is not as common
6J7 - like a EF86 sort_of but closer to a EF40 - since they both have a grid cap.
6SJ7 - like a 6J7 without the grid cap.
5693 is a high reliability/long life/low microphonic version of the 6SJ7
6SU7 is a 6SL7GT with matched triodes

don't stop there - loctals are your friends too...

7AF7 - similar to 12AU7
7C5 - is a 6V6
7C7 - is 6J7/6SJ7
7F7 - is a 6SL7GT
7F8 - is is't own beast - but somewhat similar to 6BK7
7N7 - is a 6SN7GT
7Y4 - is a 6X5GT

there are more but those are the most common types.

--DL
 

[DummyLoad]

U Da PA Man! My scores are a lot fewer and far in between  

Last one handed to me was a Bogen CHB20A but I'm still amazed that I only did a little re-arranging and got a great working amp. First one I ever got that I didn't have to completely rework to get my sound. Plate

what i've discovered over the last few years of hacking PA amps, is that the circuits with grid-leak bias sound horrible - if you have one you're time ahead to just rework those sections. i worked over a CHA20 recently minimally and the owner loves it; i believe that tubnit shared similar results with the same model.

as a general rule, i try not to disturb the circuitry from the phase inverter through the output stage.

happy hacking!

--DL

[Tyrannocaster]

I'm quite interested in this thing. I have a few 6V6 donors (all cathode biased mono amps) here and I like the way this looks. I've never used a bass control that was just a simple switchable pass filter, but the idea seems reasonable on the face of it. You said you would use different values if you were starting over with this (these point to point amps are a nightmare to refine IMO, so I can understand that) - can you try to describe why you didn't like the circuit as shown and I'd also like to hear any observations you have with respect to comparing this switching bass control  to other, more standard bass circuits.

If you have any sound clips I'd love to hear this thing too, if that's possible. It just seems like a really attractive concept.

Thanks!

[DummyLoad]

switched bass roll-off has no loss to rest of spectrum. simple high pass RC filter. the down side - it "pops" when you change switch positions. to minimize the popping, you strap 4-6MΩ leak resistors between switched poles - see schematic for clarification. most folks use a 4.7MΩ - i had some NOS 1/4 watt 5.6MΩ that worked out nice.

i attached the final schematic version - this is what's buttoned up in the chassis. BTW the rubber feet off of a scrapped cisco 6506/09/13 chassis are really nice for this kind of work and probably the best part of that appliance.

--DL


EDIT: R19 & R22 are 220K - sorry for any confusion.

[Tyrannocaster]

the down side - it "pops" when you change switch positions. to minimize the popping, you strap 4-6MΩ leak resistors between switched poles - see schematic for clarification. most folks use a 4.7MΩ - i had some NOS 1/4 watt 5.6MΩ that worked out nice.
--DL

Yeah, that was going to be my next question.  Thanks!

[tonewood]

Hey DL-
I understand the C6,C7, VR1 & Vr2 -tone / volume ckt like on some Fender tweeds but are R9,R10 & R11 tone shaping or just a voltage divider (R9:R10) and grid stopper / divider (R11: VR2)?
Thanks!

[DummyLoad]

like on some Fender tweeds but are R9,R10 & R11

are just V divider. it's 1/2 the james stack. average loss is -10 to -12dB - with the treble pot around mid-point.

--DL