Precision Electronics (Grommes) PE-15 -- Startup / Repair

[tbonuss]

Howdy,

I have a Precision Electronics (Grommes) PE-15 PA head that I am in the process of reverse engineering prior to starting up as a guitar amp. For context, I've attached some surface shots, and schematics (for "related" amps) I've gotten from Grommes. They verify that the PE-15 is one of theirs, but don't have any records of it in the digital domain to access. There may be (or very well may not be) schematics in their stacks of paper engineering documents, but the folks who know about these things have long since passed.

I've got most of the art produced for the amp as-is, and will post if there is any interest in the project. I certainly have interest in asking for help / guidance / suggestions /mentorship / jokes / harassment.

For now, I'll just post some context stuff here, then the next post ask a more specific question about cathode biasing from the heater supply...

Thank you,

Steve

[pontiacpete]

Thanks for this post!
Mine is PE 15. With the buttons. Does yours have these?

pete

[tbonuss]

So...I've been doing a lot of researching trying to document the various stages of this amp. I think I've got somewhat of a read on it, but I find this piece particularly interesting. Looks like back in the 50's, Grommes used to tap the heater supply to provide +V to the cathode for bias. I can't find any real information on this, other than tracing it back to "battery cathode bias". Any thoughts? Also, the center tap from the heater supply is not connected as they have it in the 2 Grommes schematics above, and was dangling loose without any insulation (e.g., black tape) on the wire loose end. Maybe the previous builder just used the resistors to split it? I dunno...

Thank you in advance,

Steve

[tbonuss]

Thanks for this post!
Mine is PE 15. With the buttons. Does yours have these?

pete

No--that's awesome! Are those the "cleanup on aisle 9" buttons?

Does yours have the heater supply tapped for bias on the cathodes?

[pontiacpete]

yes the heater center tap is connected to pin 8 of the power tube.

[52 Bill]

Looks like back in the 50's, Grommes used to tap the heater supply to provide +V to the cathode for bias.

I think that you have the concept here backwards. The center point of the filament winding is connected to the output tubes cathode to raise the filaments above ground by the 20 or so volts to reduce filament hum.

[tbonuss]

I think that you have the concept here backwards. The center point of the filament winding is connected to the output tubes cathode to raise the filaments above ground by the 20 or so volts to reduce filament hum.

Oh, right. I see. Thank you very much. The heater supply center tap is not connected on mine, so I guess I went down the wrong rabbit hole I'll have to re-draw it and try to understand what is going on with the added resistors and the center tap not (currently) connected, now that I'm back on track--and know what to google.

Thank you,
Steve

[g1]

The added resistors (two 82 ohm) are being used to create a virtual center tap for the heater winding. You can do it that way or use the actual center tap, but not both methods at the same time. The point where the two 82R's connect to ground could have been connected to the power tube cathode instead.
Not sure why someone would have disconnnected the stock wiring and put in the virtual center tap like that. Could be that it got burnt, or could be that they found the virtual tap reduced the hum. Does the center tap wire read low resistance to both sides of the heater winding?

[mozz]

I have some old Sams photofacts, late 40's early 50's, i may have that model or others by that company. Can we take bets how high the 3.3meg input resistor is?

[tbonuss]

The added resistors (two 82 ohm) are being used to create a virtual center tap for the heater winding. You can do it that way or use the actual center tap, but not both methods at the same time. The point where the two 82R's connect to ground could have been connected to the power tube cathode instead.
Not sure why someone would have disconnnected the stock wiring and put in the virtual center tap like that. Could be that it got burnt, or could be that they found the virtual tap reduced the hum. Does the center tap wire read low resistance to both sides of the heater winding?

Okay g1, thank you. I understand. I figure I'll just leave everything I can as the last person had it, and assume (at least for the moment) that there are reasons. Without any tubes besides the rectifier (6X5-GT), the heater coil has 7.0V across, and 3.5V both sides to the center tap. So I have no idea...maybe the Xfmr was replaced *after* the virtual tap mod...Unless anyone thinks I need to swap it back, I'll just leave it for now, and then we can try it both ways when the thing as a whole is functioning.

I'll post the updated version soonest. My intent is just to get a few of my confusions resolved, then post what I have as the schematic, and then ask if its "safe" to put the tubes in and op check it.

Again, thank you!

Steve

*later* here 'tis.

[tbonuss]

I have some old Sams photofacts, late 40's early 50's, i may have that model or others by that company. Can we take bets how high the 3.3meg input resistor is?

Thank you. That would be great!

Can I place a large bet on the input resistor value?

[tbonuss]

Here's my shot at a schematic for this. Looking forward to any feedback you may have.

From the chassis markings, this one started life with a 5Y3 rectifier, and then 6SJ7 first gain stage, 6SC7 phase splitter, and P-P 6v6s. Based on the current configuration, and comparing to all the octal pinouts I could find, its now "supposed" to have a 6X5 (existing Zenith 6X5 GT tube shorted A-K), a 6SN7 for the first 2 gain stage, a 6SN7 for gain / inversion (which was in the amp and at least the resistance measures good), then P-P 6v6's (I have a pair of new JJ 6v6S's). The multi-section (4) can cap looks to be original-ish.

PE-15 Schematic Only.pdf

Again, thank you,

Steve

[tbonuss]

So I've got most of the parts in, repaired some shorts and questionable joints--since there wasn't any negative feedback regarding the schematic, I'll assume there aren't any noteworthy / interesting problems that should preclude me from firing it up. Going through the Paul Ruby startup... So far, the PT is getting 364/360V. I've rewired the power input so that there is an on / off switch now and the fuse comes before that. But haven't put in the rectifier tube yet, because it welded the last one and I'm trying to be careful.

That old cap can had an intermittent loss of ground, replaced that 40/30/20/10 OEM with a JJ 40/20/20/20 as it is inexpensive and was easy to mount in the existing holes after I modified the clamp. Anything else I should do before firing up the 6X5 rectifier tube?

Thanks,

Steve

[tbonuss]

So, I went through the startup procedure and everything was going fine, then a flash from the rectifier tube. So, I powered down, pulled the tube, and painstakingly went through everything and continued to take readings. I finally saw, that every once in a while, there was a quick voltage transient of some kind on the big secondary of the PT. Finally, after (I think) ruling everything else out, I pulled the pilot lamp assembly apart and noticed that a small tunnel insulating grommet was missing. Replaced the lamp with an LED circuit while I rebuild the lamp assembly. AFAICT, there are no longer any spurious transients on the PT secondary.
So...in your experience is an intermittent short / arc on the pilot lamp / heater supply circuit enough to fry a 6X5GT rectifier tube supplied by the same transformer? Something fried the first one (fusing anode to cathode), and does it make sense that its likely this?

Also, the new 6X5 is a Hitachi, and I can't find any data sheets. Not sure if its also called something else, or if another data sheet is good enough?

Thank you,

Steve

[mozz]

I'm pretty sure the 6x5 was known for shorting out. If you search the antique radio forum you will find the answer.