35C5 Isolation Trasnformer - Old Westinghouse Amp

[gtrbryan]

Hello.  I procured an old Westinghouse Amp with uses 35C5's as power tubes.  Of course, those tubes have 35v heaters and are often used in cheaper amps without a PT/Isolation transformer.  Fortunately, this amp has an isolation transformer stock.  Of course, I want to install a fuse and three pronged cord.  Here is what is confusing to me.  One of the secondary taps goes to ground as well as the the heaters.  Will a three prong cord, with the safety ground, cause a problem here?  It just isn't what I'm used to seeing.  I found this source about adding an isolation transformer and it seems to say this is the right practice, but it seems odd to me.  http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/step4/Parts-and-tools/  So, I thought I would ask the experts here.  Schematic attached for reference.

I will obviously have to rewire the preamp, so any thoughts are welcome as well.  The voltages are low (120v on 12AV6) so I am wondering if I should change the stock 47k plate resistor to 100k or not.  Thanks as usual.

[sluckey]

One of the secondary taps goes to ground as well as the the heaters.  Will a three prong cord, with the safety ground, cause a problem here?

No problem.

I will obviously have to rewire the preamp, so any thoughts are welcome as well.

Make it look like a typical blackface Fender preamp.

[DummyLoad]

you'd ground the CT of a full wave PT. the shield is grounded as well on transformers with a winding shield - usually the orange wire on a fender transformer. add your grounded line cord and tie it to the same ground point as the shield ground if possible. this is a half wave PS - it's going to hum unless the OT rolls off at ~100Hz. i suspect it does.

R11 (68k) connected to 120V doesn't seem right. but then again 68k/680r is 1/100 or 1.2V with 120V B+; add Ik loading with 470 rk, then bias sits at ~1V.

if you have to "fix" the 12AV6 stage - get rid of the 1m & 330k input resistors, the coupling cap, and 10m grid leak. leave the 47K and add 1.2k-1.5K cathode R and bypass with 2-10uF for for more gain in needed. use a 22k-68k grid stopper and 1m for the grid leak. think fender/marshall input stage.

with 47k plate and 1.5k cathode - no bypass gain is about 15, bypassed around 30
with 100k plate and 1.5k cathode - no bypass gain is about 25, bypassed around 50

with 120V B+, input threshold is going to suck if you "fix" 12AV6 input stage. with hotter pickups this amp is going to distort easily and maybe that's what you're going for.

is this really 1/2 of a stereo amp? is R1 a dual ganged pot, or is this a mono amp and R1 is really just a volume control?

love the low budget x-over network for the mid/tweeter.

--DL

[gtrbryan]

From what I can tell, it is just a mono amp and "balance" is just volume. 

I'll make those changes and see what happens.  Thanks to all.

[PRR]

> From what I can tell, it is just a mono amp and "balance" is just volume.

Stereo records were coming. Mono recordplayer sales stalled, buyers waiting for stereo.

So they sold you a good mono player *with* a jack for a second channel. When you finally bought a stereo record, you bought the matching second box to make it STEREO!

I suspect there was a stereo volume pot in the main motor-amp. Since the main and add-on amps were not matched, and room placement could vary, the "balance" knob allowed the user to trim out unbalance. Once set it would rarely be touched. (I also suspect they used the same chassis for main and add-on, and the bal pot filled the hole where the Volume pot was in the main amp.)

[smackoj]

I've had a couple similar amps in the "all voltages thru the 3 or 4 tubes equals 120 volts" category. I had a cool Japanese stereo amp like that but, at that time, I wasn't a member here so I moved it on down the line without attempting a re-build. I didn't have the knowledge to make a working guitar amp out of it.....those kind of amps are still out in the shops and garages in big numbers IMO, so a guy could find several if he was looking. I'll continue to follow this thread and a few other similar ones to keep my head in the game until my fingers can catch up.
 

[HotBluePlates]

For what its worth department.  Find two 6av6's and a socket, replace the 12av6 with a 6av6, and use the other 6av6 as your preamp.  (Wire the heater filaments in series to replace the 12av6).  Other choice, take another 12ax7, and its socket, and replace the 12av6. (Wire the heater for 12v not 6v (some rewiring required).  You now have a extra triode for your pre-amp.

He has to use the 12AV6, or the 12AX7 wired for 12.6v.

Each of the tubes in that amp have 150mA heaters, but the 6AV6 (and 6.3v 12AX7) draw 300mA.

Can't get there from here with 6AV6, but as you note, 12AX7 using pins 4 & 5 as ends of the heater will land at the right spot for both voltage and current.

[HotBluePlates]

The issue is a series heater string has to use tubes that all draw the same current.

Or, if he could find tubes that draw 75mA of heater current, he could parallel 2 of them to match the 150mA drawn by the rest of the string.

But dropping 108v or so through a 720Ω resistor passing 150mA will add 16w of waste heat in the chassis (and require a 30w or bigger resistor). The PT may have some reserve current, but almost certainly not double-current, especially in a series-string heater amp that's all about pinching pennies.

[gtrbryan]

Interesting development. I got the amp in the mail and it turns out instead of a metal chassis the tubes are on a circuit board.  It's very old, but I guess it's still a "printed" circuit board of sorts.  Will make changing tubes out pretty difficult. I may be stuck with the 12av6 in the preamp, grid leak bias.  I may use the first stage of a 5c3 and see what happens.  Any thoughts?

[DummyLoad]

gut it!

 

a friend gave me an amp chassis out of a scrapped sears silvertone organ. came to me as a PCB amp as well 6AQ5 single tube class A power amp. the amp i had a came with a more conventional power supply, but don't let that deter you.

link to metamorphosis pics below.

http://s2.beta.photobucket.com/user/pmitchel/library/Brocks-Organizer

--DL

[gtrbryan]

I put some fuses on it and fired it up this weekend.  It sounded just fine unmodified with my iphone playing through it.  When I played guitar through it it sounded awuful (whcih is expected).  I was able to change the preamp to a 5c5 type, grid leak bias.  Sounded awful.  Lots of humm.  Took the circuit board out and this is going to be such a pain to work on I think I will just take your advice and gut it.  Putting in a different tube, or just getting rid of grid leak bias would be a mess.  I have a few ideas and will let everyone know what transpires. 

[gtrbryan]

Now that I've decided to gut this circuit, I'm trying to figure out what to do with it.  I have an Isolation Transformer, so I'm thinking of using "those" kind of tubes.  I ran across the "All American 4" in Building Vacuum Tube Guitar & Bass Amplifiers by Tino Zottola.  I would need a new OT, but my PT should work fine.  Has anyone built this amp?  I could find no clips on the net for sound.  I may substitute a 12AX7 in V1 so I have an extra triode if needed.

[DummyLoad]

shouldn't have to do much, but then again that's relative... see attached schema - it's the all american 5! keep all your iron, a couple of minor changes to PS to accommodate the additional 12AV6 heater; i changed the value of R18 in the original plan. R19 in the new plan will probably need to be tweaked (smaller) as well.

if you want more @$$ out of it, then bypass R16 with 47-100uF. if it's too hot, use 100K for VR2 and double the values of C4 & C5, or use a resistor divider network.

--DL

[gtrbryan]

Wow. Thanks!  I can't believe I missed that.  It's so close to my stock amp. I think think I'm going to gut the printed circuit board. This way I can use all my old iron and tubes.

One question on the series heaters. Is pin 9 on the 12ax7 included?  I thought in series you just use 4&5. Thanks again.

[DummyLoad]

Is pin 9 on the 12ax7 included?  I thought in series you just use 4&5

pin 9 is not connected - string uses 4 & 5.

--DL

[PRR]

> the "All American 4"

Your record-player is already sigNIFicantly better than a cheap radio. Why downgrade?

From the Balance (volume) pot right, your circuit is fine. Paraphase driver is an old-old-old standard. 35C5 runs lower voltage and higher current than some other tubes, but you GOT the transformer to make it come out at speaker volts/amps.

You want one preamp stage and a low-loss tone network; or a heavy tone scheme and two preamp stages.

With only 120V B+, that first stage can NOT have large voltage gain or a hot geetar will overload it. 120V will support 16V out, guitar jack should accept 0.5V-1V gracefully, gain should be 15-30 not the 45+ of a full-out 12AX7(12AV6) stage. I'd ponder 12AT7, because it can be shimmed to this gain range, is readily available, and it WAS popular in radios (as the RF stage in FM radios).

[HotBluePlates]

Or 12AX7 first stage with no bypass cap (local negative feedback drops the gain to 30-35).