Old Time 50's Octal Tube Preamp Needs Review

[smackoj]

Ola Amigos, Feliz Navidad; I am building a simple preamp using some Old Time Octal Tubes. At this moment I am using a schemo I found on the Net. I have a 6AX5 rectifier and a 6SN7 dual triode preamp tube.
My questions: Can someone look at this schemo and tell me if this circuit is Poor or Good or Great?
I have other tubes that I see used in other old amps like 6J7 6SC7 6SL7 IF these might work better?
Is there a good purpose for there to be a Polarized Cap next to the Output? Will an Orange Drop work as well?
Does this circuit produce an impedence that will plug into a guitar amp without problems?
Thanks friends for you help,  Jack D  Arizona

[kagliostro]

Despite the Two Rock 35 has a pot in that position (higher value - controverse pickup load boutique feature), usually no guitar amp has such a configuration

As you can read on the schematic that pot Is labeled as dual audio and from that you can argue It Is part of a stereo configuration (HiFi I suppose)

So don't think it is  a great circuito for Guitar input use

There are lot of old guitar amp from which you can borrow an octal tube input, try among Gibson schematics if you find someting you like

---

I read you want to use the circuit betwen Guitar and the amp input ? What do you want to achieve, an overdrive pedal ?

Franco

[mountainhick]

What do you want to accomplish with this?

6SN7 triodes have nominal amplification factor of 20, do not provide a heap of voltage gain which is generally what you want for a guitar pre amp. They provide a good amount of current as drivers/phase inverters.

The circuit is a cathode follower output/line driver. That is not useful other than to drive a tone stack or as a buffer to drive a long cable or low impedance to line level effects.

You mentioned 6SL7 (discrete cathodes), and 6SC7 (Shared cathode). these are higher gain, af of 70. Better choice iMO for guitar preamp gain stages.

Are you referring to the 0.47 cap? That is necessary as a (de)-coupling cap which blocks high DC voltage to the output but allows the AC signal. Yes, orange drop is fine as long as sufficient voltage rating

[smackoj]

Merry Christmas and thank you all for your replies;"What do you want to accomplish?"
I have lots of vintage tubes, trannies and a few usable chassis lying about so I thought I would use them.
I also have a few vintage solid state amps that do not have a Preamp section.
I'm want to see if I can build just the Preamp using vintage octal tubes. I am hoping to get a more vintage tone without spending money on new parts. High gain is not the object.
Thank you for all your help amigos

[bmccowan]

Merry Christmas to you too. I have built and rebuilt many amps with octal preamps. I second Kagliostro's suggestion to borrow ideas from existing amps/schematics. In addition to Gibson, I have had good luck with 1940s and 50s preamp circuits from Rickenbacher, Valco family (National/Gretsch/Supro), Magnatone, Ampeg, Silvertone branded amps from Valco and DanElectro, etc. Of course there are also early Fender amps that are all octal. There are tons of these schematics right here in Doug's schematic library.
There are many concerns out there about microphonics, but if you are not going for high-gain, there is no big issue IMO. Added bonus - octal sockets are much easier to solder.

[smackoj]

Thank you BMC; sorry for a slow reply, I thought my Post was idle and no one responding.
I couldn't get where I want to go with a Preamp so I decided to add a 6V6 output tube and see if I like an actual amplifier. As you note, there are lots of early Octal designs in the Schemo Library. It's an old Valco Clone chassis being reused so it ain't pristine but I don't have good enough tools for punching socket holes so reuse is the program for now. I will try to follow with pictures and sound bites but still don't have it sounding top floor yet.
Jack D

[clayman]

I had an old Bogen that I modified for guitar and used 6sf5 that is just half of an ax7. It had 6c8 for phase inverter and tone stack. The 6c8 and 6F8 sounded really good and had top cap plates for cool factor.
 I might use them again with a magic eye if I can find one.

[bmccowan]

Thank you BMC; sorry for a slow reply, I thought my Post was idle and no one responding.
I couldn't get where I want to go with a Preamp so I decided to add a 6V6 output tube and see if I like an actual amplifier. As you note, there are lots of early Octal designs in the Schemo Library. It's an old Valco Clone chassis being reused so it ain't pristine but I don't have good enough tools for punching socket holes so reuse is the program for now. I will try to follow with pictures and sound bites but still don't have it sounding top floor yet.
Jack D

You shouldn't need any more socket holes in that chassis. You have enough to make any number of designs that do not have reverb or tremolo. You mention adding a 6V6 output tube. If that power tranny was running two 6V6 power tubes, per the markings on the chassis, it may be a bit much for a single ended/single power tube amp. You may want to take a look at the Fender 5C3 Deluxe schematic to see some possible directions, and then look for similar Valco amps if that's your flavor. Nice thing about Fender docs is that you typically get both a schematic and a layout.

[smackoj]

Hello again friends; I have been away for a week so here to catch up and ask a few more questions; I gave up for now with the octal preamp idea and switched it to 12A_7 preamp tube. The more I tried to refine the octal preamp section, the more problems I seemed to create.
I still want to do the single ended output tube and going with one 6L6 because I switched PT out and the new one is 400+ vdc.
Question: I don't think I have an output tranny for a 6L6 SE single ended? Can I use a PP tranny with modifications?
2. Is there a proven method to use an IC chip to do the phase inverter task with 2 or 4 output tubes?    I might change to a PP with 2 6V6s or 6L6s and would like to a chip instead of a tube for Phase Inversion. 
Adding new picture of same chassis with 9 pin sockets added.

[shooter]

I don't think I have an output tranny for a 6L6 SE single ended?

what SE OT's do you have?
using a PP ot for SE is poor design, possibly exciting though if she smoke playing "smoke on the water" 


the cost of a 2nd tube, socket, pi n all the pieces parts is probably less than a basic OT

[smackoj]

I don't believe Amp smoke on water would be 'exciting' in a good way?!
I only have one SE OT and it's for a single 6V6 or EL84 tube.  I thought I would switch to 6L6 because the B+ is over 400 volts. I'm not sure but I would guess this one small OT would not like that much voltage either?
I thought about adding a 2nd Pwr tube but then I am not even doing a Single Pwr tube amp. Thus my question about adding a phase inverter using an IC chip. Maybe that's silly, I don't know?
Gracias Amigos

[smackoj]

I think I will scrap the SE idea, rig the amp for 2-6L6s and avoid voltage issues. Seems like the quickest way to 'finish' an amp for me. I have several PP OTs so no more searching for main parts.
Gracias Amigos

[AlNewman]

How did you determine your PT is over 400V B+?  Is that under load?  It may be that the PT would work very well with either 1 or 2 6v6/EL84 tubes.  It could also be that it would work well with either 1 or 2 6L6/EL34 tubes.
Do you know where the transformer came from?  If so, it might be best to design your amp to roughly follow that schematic.

[smackoj]

I checked the rectified power and it was at or above 500vdc NO LOAD
I was thinking it's too hot for the small SE OT which originally ran off ONE 6V6
The PT is vintage and lists a 5Y3 rect. tube and a couple 6V6 power tubes so it seems lower voltage under load of at least 2 power tubes?
Thanks for your advice. Jack

[AlNewman]

So what's the model number?  Do you have the original schematic?
They weren't so dumb in the old days, they would design amps based on transformers rated for heater and B+ current draw.  It wouldn't surprise me if under load you would be well under 400V with 2 6V6's at idle, especially with cathode bias.  It also depends on what it was used in, but with a 5Y3 rectifier, it probably wasn't an organ or anything.

[bmccowan]

Smackoj - I will politely suggest that your approach is scattered. IMO building tube amps requires a thoughtful, organized, approach. Otherwise what should be an enjoyable experience becomes a frustrating exercise. I suggest you follow a known successful approach, such as:
 - Take stock of what you have on hand
 - Decide what you want to build
 - Find a known schematic that fits
 - Or; draw a schematic and check with experienced people here or elsewhere for sanity checks
 - Purchase what you need
 - Depending on your level of experience - read up and/or watch videos
 - Build step-by-step checking with those experienced folks along the way
Ignore this if you want - its just what has worked for me and others here.

[smackoj]

Yes, good advice. I have a design that is simple enough and I have all the parts I need like an OT.
The only SE OT I have is one for 6V6 or 6BQ5
Gracias Amigos