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

[nosaj]

I'm pretty sure the 6x5 was known for shorting out. If you search the antique radio forum you will find the answer. The filament shorting should have nothing to do with the high voltage though.

Are you running a separate filament for the 6x5 or is it sharing the same 6.3 filament as the other tubes?

Nosaj

[tbonuss]

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

Thank you Mozz! Thats a good site. I read a bunch of stuff about the 6X5 GT and I guess its not the most highly regarded tube series, and the original Zenith seems to be the biggest offender. Lots of back and forth about whether it was because Zenith was overloading it powering 8-10 tubes. Seems like the issue was a spiral filament that arced "easily" to the cathode...but, lots of back and forth on that too...maybe I should change this out eventually? Or just put in a 6AX5? Lots of back and forth there on that too.

Were you able to find any schematics for this? That would be really helpful.

Thank you,

Steve

[tbonuss]

Are you running a separate filament for the 6x5 or is it sharing the same 6.3 filament as the other tubes?

Nosaj

Thank you for the question Nosaj. All the heater filaments and the pilot lamp are powered by the same winding of the PT secondary.

Thank you,

Steve

[tbonuss]

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

Hey Pete, would it be possible to tell me if your Power Transformer is marked TP - 9U? The engineer at Grommes says he thinks thats right because the PE-10 used their TP - 7U and this is a little bigger...he doesn't have any records on the PE-15.

Thank you,

Steve

[pontiacpete]

This one here is a TP2U.

pete

[mozz]

Are you running a separate filament for the 6x5 or is it sharing the same 6.3 filament as the other tubes?

Nosaj

I always forget that the 6 volt rectifier usually shares the filaments with the rest of the tubes. Get so used to 5y3's and when you see a 6ax5 or 6x5 it doesn't sink in.

[nosaj]

I always forget that the 6 volt rectifier usually shares the filaments with the rest of the tubes. Get so used to 5y3's and when you see a 6ax5 or 6x5 it doesn't sink in.

Not sure if this applies here or not but this is what prompted my question. Link is here Antique Radio Forums ? View topic - 6ca4 rectifier questions

The 6CA4 heater is grounded. Cathode, which is close to the heater, will have high DC. Data book spec shows 500 as the peak limit. If the tube should arc over something has to burn up, power transformer?

You could use a separate heater winding, which isn't grounded, for the 6CA4.

[tbonuss]

Lots of good stuff to read / think about. Thanks Mozz and Nosaj.

Seems like there are a lot of reasonable options including adding a separate heater xfmr, and running a different rectifier tube. The 6AX5 also seems reasonable almost as is, but it specs at twice the filament current, and I don't have the specs on the xfmr....so some risk there unless I add an xfmr.

Another option is to simply use the Raytheon JAN 6X5WGT...from what I read on the Antique Radio Forum, this seems like the most straightforward approach until I get this running. Then maybe mod later... Make sense?

[nosaj]

Lots of good stuff to read / think about. Thanks Mozz and Nosaj.

Seems like there are a lot of reasonable options including adding a separate heater xfmr, and running a different rectifier tube. The 6AX5 also seems reasonable almost as is, but it specs at twice the filament current, and I don't have the specs on the xfmr....so some risk there unless I add an xfmr.

Another option is to simply use the Raytheon JAN 6X5WGT...from what I read on the Antique Radio Forum, this seems like the most straightforward approach until I get this running. Then maybe mod later... Make sense?

Between here and the antique radio forum the help and knowledge is far and wide reaching.

nosaj

[mozz]

Looking at that schematic, you may be able to get by with a solid state rectifier, it's only showing the filter caps at 400-350v? JJ's can take a bit more than other brands. Also, if you take out any 47 lamp and use a l.e.d., that will save you 150ma on the filament current. I didn't know the 6ax4 takes 1.2amps. Also, the JJ's might draw a little more than a old 6v6. If you get it running, be sure to see how hot the power transformer gets. The only Grommes schematic i can find in my stash is a preamp.

[tbonuss]

Looking at that schematic, you may be able to get by with a solid state rectifier, it's only showing the filter caps at 400-350v? JJ's can take a bit more than other brands. Also, if you take out any 47 lamp and use a l.e.d., that will save you 150ma on the filament current. I didn't know the 6ax4 takes 1.2amps. Also, the JJ's might draw a little more than a old 6v6. If you get it running, be sure to see how hot the power transformer gets. The only Grommes schematic i can find in my stash is a preamp.

Thanks Mozz!

I've thought about plugging in one of the Weber solid state "tubes", but I thought I'd learn more if I keep pounding away on this--I may break things and learn hard lessons...I am trying to avoid it, but I know its likely.
I need to update the schematic--I've added a few things, and changed the caps to 500V JJ, with 40/20/20/20.
The datasheet I'm looking at says 1.2A@6.3V for the 6AX5-GT.
I'll make sure I monitor the temp on the power transformer. I got the JJ's because I didn't know if it was 6V6 or 6L6...and (from what I read on the internet) they are kinda sorta' in-between. Mistake?
Thank you for looking for the schematic.

I pieced together an old PC from stuff in the basement and downloaded the Duncan P/S Simulator software. On my first rough attempt it looks like I'm drawing a tad too much current for the 6X5GT's. When I get a chance this weekend, I'll fix the schematic from posts above and also see if I'm using the software correctly. I made some assumptions on the caps, choke and load resistance that were just swags.

Thank you for taking the time to help,

Steve

p.s., Does the amp look like anything you've seen before? I can't really map it on to anything, to figure out what the guy was trying to do , but I'm just googling and staring

[tbonuss]

Finally had a chance to get back to this. Added a few minor things and updated the schematic (e.g., 500V caps, discharge resistor, some LEDs to monitor P/S cap charge/discharge, some #47 lamp "fuses" for the P/S Xfmr secondary because of the 6X5 issues). P/S checks out good on the Duncan Simulator. Getting steady 480VDC out after about ten minutes of run time. Nothing flashing or overheating. When I get a chance this weekend I'll pop all the rest of the tubes in and hope for the best.

PE-15 Schematic Only V2 02FEB17.pdf

Thank you,

Steve

[tbonuss]

Things are working pretty well. The lamps combine to take about 5 volts combined off the top--which I think is a good thing.

I'm getting 326 / 328 at the anodes of the 6V6's, 125 / 124 at the first gain stages (6SN7) and 75 / 72 at the phase splitter section (6SN7). No red plating or xfmr heating. Its a bit small sounding at low volume, but has a nice overdriven sound (I dunno, AC/DC Powerage-ish). Its into my beater "test" cabinet, a 15" Trace Elliot.

Based on the LEDs, its drawing a little more current at idle than I expected, but thats just qualitative as I haven't biased it yet.

Something is making a *very* faint metallic clink sound every once in a while, and a bit more on cool down. Seems like its just heating and cooling but I don't have enough experience to know if its an indication of something else. No flashes, voltage spikes or noise that I can tell.

After the amp is good and warm, the last cap discharges on startup dropping from 80V to about 50V about one out of every five times, then it catches its breath, charges up and everything starts just fine. Maybe the 6V6s are drawing earlier when they are warm (?) I dunno.

Also, the hum is way way less than I expected. Whoever converted this thing seems to have really know what he was doing.

All good so far.

[tbonuss]

Screens were +13/+12 to plates, so I put in the traditional 470ohm screen resistors. Down to about +10/+9 for now. Never gets less that +5 with everything dimed and "exaggerated" input signal (quickly and repetitively banging chords on the guitar).

Plates on 6V6s dropped about 4 volts. Lamps on the Rx tube plates down to about 4.5 volts combined vice 6--that's better.

Didn't expect that, but I also redid half a dozen solder joints where a previous tech had basically "veneered" old corroded solder joints with new solder. I guess he was trying to redo them...took them all down to bare metal and redid them. PITA.

Running Class A. Boring unless dimed with exaggerated input as above. Sound is a little bigger at low volume with the screens less greedy.

[tbonuss]

Pulled the choke and replaced with 2.4K power resistor. Dropped the screens to -10VDC from the plates at idle. Much better. Starting to have some character.