Grommes 50PG into a guitar amp

[Cygnus X1]

http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN:IT&ih=005

A friend sent this one to me.
He wants it gutted, and built into a "guitar amp".

It has 6 octal sockets. It was a 10 watt monoblock.
I have no idea what the primary impedance of the output transformer is.

I was thinking 5C3, Tweed Deluxe...with 6SC7 preamps,
2x6V6, 5Y3 rectifier. I would leave that 6th octal slot empty.

One issue is I want to use those bass/treble controls as-is.
I don't know how to wire a tweed "tone" control into "bass/treble". I glanced at the Pro-Amp, the Bassmans, but it looks like using the combination of the octal 6SC7's is just something they didn't do.

The phono/radio switch on the far left could be an internal jumper for a single jack system. I plan on using just one jack, one volume control.

Any ideas here?

And any clue how I could use the existing output transformer?
I think the PT would be sufficient.

Thanks, and have at it!

[Cygnus X1]

:Snore:

OK 5C3 it is.
Now...how do I figure out if the output transformer will be ok?

Edit-the OT will be fine.

Can anyone tell me how the 5C3 circuit sounds (compared to a more familiar 5E3?)

[steve25]

That OT should be fine just the way its, in fact if I recall right it was a interleaved wound model.
that speaker socket takes a regular Hammond organ plug that you can get from Antique electronics supply and you`ll be on your way to using it.

[Cygnus X1]

Thank you, Steve.
I have some organ parts from a Balwin that I picked up.
I will likely use one or two of those RCA jacks for 4 and 8 ohm outputs.

The guy I'm doing this for sent me this for payment in advance:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=230328582176

Very nice!

[ziess]

Cool project! Is that four or two 6V6GTs in the amp originally?

[Cygnus X1]

Tube compliment was 5Y3, 2x6V6, 2x6SC7, and a 6SQ7.
I'll be gutting it, but it looks like the SC7's were the preamp inputs, maybe the SQ7 was for PI, and of course the 6V6 for the power tubes. I never did find the actual tube placement and they want money for the original schematic. No sense in buying it. (Grommes is still in business).

Given the limited options, and the "cool factor" in the appearance-I will put a single input where the selector chicken head pot is. Dual concentric (independent) pot for the two volume knobs-internally jumpered. I sort of did this with my own 5E3 build. I used 2 inputs instead of 4. I just use an
external jumper config using a 2-to-1 Radio Shack adapter.

Still wondering about the 6SC7 type 5C3 tweed circuit.
I'm hoping those tubes will be up to snuff.
Never played one, or knew anyone that has.
So I'll be trying this "blind".

[ziess]

A couple of guys have built them on the Weber boards. Perhaps you could contact them.

http://www.weberorders.com/forum/index.php?topic=372.0

[Cygnus X1]

"Membership Required".

I need to fix my email issues to get into other sites.
It's a whole 'nuther story.

[ziess]

Do you have a working email address?
I could email you a PDF of the page.

[Cygnus X1]

I get it to you.
Thank you, ziess.

[Cygnus X1]

Oh...fudge.
The 5C3 schematic calls for 16uf on the filter caps.

I have a bucketload of 10uf/500V.
Do you suppose that is close enough?

[Cygnus X1]

Arf. Arg. My brain hurts.
Help me out please...

I want to use that bass/treble control on the Grommes and incorporate it into the "tone" control for the 5C3 tweed.
So I looked at the 5F4 schem, and came up with this:

Treble lug 1 runs to 6SC7 #2, pin 2.
Treble lug 2 runs to 6SC7 # 2, pin 4.
Bass essentially goes to ground through a tone cap.

One of the issues I'm dealing with is one is a 12AX setup, and I'm using 6SC7's for preamp tubes.

Check the 5C3 schem, and see if that seems correct before I Kurt Cobain myself here.

http://www.turretboards.com/layouts_...deluxe_5c3.gif

https://taweber.powweb.com/store/5c3_layout.jpg

5F4:
https://taweber.powweb.com/store/5f4_layout.jpg

[ES350]

I have a '54 with 6SC7s, and that's the schem...although I have seen other stock 5C3's with 12**7 pre's. I like the 6SC7 sound; fat, less gainy, a little darker like the earlier tweeds.

[cdw2000]

Oh...fudge.
The 5C3 schematic calls for 16uf on the filter caps.

I have a bucketload of 10uf/500V.
Do you suppose that is close enough?

The Grommes Little Jewel is a nice little monoblock and has good conversion potential. Are you sure you want to gut it???

If you have room you can use two 10uF in parallel at each filter cap point to get you 20uF. Or do 20uF at the first filter cap only and 10uF for all the others.

[Cygnus X1]

Thank you, ES.
I'm building it, and looking forward to comparing it to the 5E3 I already built.

I still haven't quite puzzled out how to connect the Bass/Treb instead of a "Tone" control. It either goes directly into the pins mentioned, or it gets tied in at the same point as the "Tone". Live and learn...I will note just where the smoke comes out!

[Cygnus X1]

The Grommes Little Jewel is a nice little monoblock and has good conversion potential. Are you sure you want to gut it???

If you have room you can use two 10uF in parallel at each filter cap point to get you 20uF. Or do 20uF at the first filter cap only and 10uF for all the others.

The fella that bought it wants it converted. He already has sweet monoblocks. (Dynaco's). So given the go-ahead, the only thing left original will be the chassis, the faceplate, and the transformers. Everything else will become a 5C3...with the exception of the above mentioned Bass/Treble circuit.
Any help there is appreciated.
I'll even take motion pictures of the smoke if necessary.

[Cygnus X1]

I would never make a third post to myself, would I?

No...not me.

Fine. Leave me out here in the snowbank to learn for myself.
It won't make any "smoke" if I screw it up, just won't make any sound, and I'll be scratching myself like it's flea season.
And I won't tell anyone what I discovered in the self-discovery!

Have a great weekend, I plan on doing so.

[cdw2000]

I'm not able to visualize exactly what you are attempting to do with the treble control lugs. I would need a schematic.

In any case I don't think what you are proposing would work unless you add a third tube stage. The 5F4 tone stage comes off a low impedance cathode follower stage and feeds into the phase inverter. You would need to replicate the cathode follower stage of the 5F4 in addition to the tone controls.

[Cygnus X1]

I went by the schem to get it built, just a tone control for now.

It is in break-in right now. It has tone very similar to my other Tweed Deluxe project, the 5E3.
But two things I have noticed right away. Lower overall volume, and less headroom. So I think I am going to play with the dropping resistors on the preamp tubes, and look closely at the bias on the power tubes. Correct me if I'm wrong about what to do with the preamp tubes. I'm not going for high volume with it.

I tried some 6V6 GTA's, and it likes older Sylvania GT's the best.
The GTA's seem to run too hot and break up, but not in a good way. (fizzy). I tried a few different 6SC7's, and it didn't change the tone much at all. The Fisher 5Y3 recto is solid. I also swapped in and out RCA, GE, Tung-Sol. No difference in "sag", and didn't show any significant drop on the meter under load, so it stays.

I never did get a primary impedance value on the output transformer, but I suspect it is a soft, conservative value for hi fi use. It warms the amp up quite a bit (sonically-it "runs" cool).

It is very quiet, hum free.
I haven't learned much, but I know how to run grounds, and not rely on the chassis. I could build these things in a cardboard box!

I'll spend some more time on it this weekend. It promises to be really sweet!

[cdw2000]

Hi,

Since the Grommes was only a 10W amp, the B+ voltage out of the PT may be lower than for a standard 5C3. I searched the web for a 50PG schematic but came up empty. I was hoping to find some voltage values.

Though they are hard to read, page 2 of this 5C3 Fender schematic/layout shows voltage measurements referenced to ground.

http://www.schematicheaven.com/fende..._5c3_schem.pdf

It looks like the original 5C3 had about 280V on the B+ side of the 250K plate resistors and 85V on the plates of the 6SC7s (pins 2 and 5). So you should measure those points on your amp for comparison.

[Cygnus X1]

Yup.

Guess who didn't have any 250K's, and used 220K's...and called it "close enough"?

My voltages are running a bit higher.
300V, and 95V, respective.

[cdw2000]

Actually, 220K's probably should be "close enough" for now. The ratio between the plate resistor and cathode resistor helps determine the gain of the stage (up to a point). So by using 220K resistors, you just lowered the gain slightly.

I was more interested in the B+ voltage on the high voltage side of the 220K resistors. This determines the maximum voltage swing or "head room" that is available from that stage. If the voltage is too low, this can limit how much you can drive the next stage.

Based on your voltage readings though, that should not be the problem. 300V and 95V is in the ballbark, but if you wanted to you could increase the value of the 10K resistor between the last filter caps in the high voltage supply chain (the last one supplying the high voltage to the 6SC7s). Maybe try a 12K or 15K resistor if you got one.

Did you make any other value substitutions?

[Cygnus X1]

I'll double check everything and look at the B+ to the 6SC7's.
That was kicking around in my head when I was taking readings anyways. A consensus spells an idea worth trying!

G. Weber says this animal is very sensitive-I believe at least on this one he is dead on. Not so much on some of his other ideas.

Thank you, C.

[cdw2000]

I replied to your other thread. Have you double-checked the bias current for the 6V6's? If the OT is not presenting the correct impedance to the power tubes (relative to the 5C3 OT), then the tubes operating point will be shifted and you may need to select a different cathode resistor to get the bias current right.

That fact that the B+ voltage is higher should give you more headroom when everything is dialed in. The fact that you mentioned some tubes have a "fizzy" sound indicates that you maybe running more in Class B mode rather than Class AB as the fizzy sound maybe cross-over distortion. Does the fizzy sound stay at a fairly constant volume as the main signal decays? That would be cross-over distortion. Measure the plate voltage and use that for determining the required bias current.

Use Weber's bias voltage calculator to determine the new bias current. Measure the voltage across the cathode resistor and divide that by the new desired bias current to determine the ballpark for the new resistor value (the bias voltage will also shift with the new cathode resistor, so this only gets you into the ballpark). Select the next larger standard value. Re-measure the plate voltage as that can also shift upward when you reduce the bias current. Recalculate the target bias current again using the Weber calculator, then compare to your new measured bias current. The current should be at or slightly below the Weber value, never above. All these parameters are inter-related so some trial and error is involved.

Good luck and enjoy.

[Cygnus X1]

Fuzzy gut shot...not much room to work, but I went with using boards anyways.
I thought about doing PTP like I did the Spitfire, but this worked out great.

Gris was talking about a tight build-I got the entire preamp section in the width of two octal sockets.
The third portion is the another two filter caps and the bias resistor for 6V6's.

I put the main filter caps transverse on an elevated Lexan board.