Conn Organ tube amp, 6L6G. Any recommendations for a new build?

[turbofeedus]

Picked up this chassis from the same person who sold me the Hammond AO-63 chassis.
Looks to be about 1952, at least that's what I gather from the can caps and CTS pot.
Drew up a schematic:

schem1.pdf

The good:
I thought the crud on top was rust, but it's just amber splatter on top of the gray paint. It cleaned up fine.
The price was alright IMO, $100 for the chassis and tubes.
Plenty of space in this chassis for internal filter caps.

The bad:
May have been hosed. Found a jumper in one of the fuse holders.
The purpose of the second PT, and specs on the OT are, at the current moment, a complete mystery to me.
I rubbed off the 6L6G logos like a doofus.

Unfortunately I didn't find the fuse jumper until I already gutted the amp, so I can't really fault test the original amp.
Hopefully it was a shorted cap or bad tube, and not an issue with one of the transformers.

I could have sworn I saw a schematic for a Fender amp with 6L6G, am I delusional?

connamp-6.JPG

connamp-1.JPG

[xtian]

I built a nice 5E3 out of a Conn organ donor. Came out very nicely. Here's my build log: https://ampgarage.com/forum/viewtopic.php?t=29842

[thetragichero]

as long as the transformers are good i wouldn't say you've been hosed. chassis, transformers, and maybe tubes sounds like an okay deal (more than i would pay but I'm also scouring Craigslist for almost half the state picking up organs for free)
6l6g are interesting. I've used in a 5e3-type amp and just finished a vibrochamp-style build running a 6l6g with a bigger output transformer. they look cool as hell

[turbofeedus]

I built a nice 5E3 out of a Conn organ donor. Came out very nicely. Here's my build log: https://ampgarage.com/forum/viewtopic.php?t=29842

Thanks I'll check that out. I saw a couple Conn amp projects around, but none with this chassis.

as long as the transformers are good i wouldn't say you've been hosed. chassis, transformers, and maybe tubes sounds like an okay deal (more than i would pay but I'm also scouring Craigslist for almost half the state picking up organs for free)
6l6g are interesting. I've used in a 5e3-type amp and just finished a vibrochamp-style build running a 6l6g with a bigger output transformer. they look cool as hell

Fingers crossed. Going to test them soon.
I knew I had seen some Fender schematic with 6L6G, might try this:

Fender_bassman_5e6a_schem.pdf

[turbofeedus]

Did some testing on the two transformers, to see if the fault lied in there, hopefully not.
Measurements done with my Fluke 179

PT
I applied 4.593V across the primary, alternating between the three primary leads and black, then measured across the HT on the secondary.
green/black = 29.74V
black/gold = 27.44V
yellow/black = 32.43V

I'm imagining these were different taps for 110/115/120V ?
I also measured the 6.3V secondary winding, and the lowest voltage (black/gold) primary tap yielded 6.8V. I will probably go with that unless I need more HT voltage for some reason.

OT
I applied 4.796V across the secondary, alternating between the three secondary taps and common (black), and measured the voltage across the primary.
yellow = 192V = ~40:1 turns ratio
green = 104V = ~21.5:1
gray = 139V = ~29:1

If I interpret yellow to be 4Ω, gray to be 8Ω, and green to be 16Ω, that would yield:
yellow = 40x40x4 = 6400
gray = 29x29x8 = 6728
green = 21.5x21.5x16 = 7396

Which all seems to jive according to 6L6G datasheet.

One thing that concerned me, and maybe it's nothing, but I was drawing some current when performing these tests.
I recall the test on the green* tap for the OT specifically was drawing ~160mA according to the meter on my variac.
Is this normal? Just my multimeter loading it down?

*Correction: yellow

[xtian]

One thing that concerned me, and maybe it's nothing, but I was drawing some current when performing these tests.
I recall the test on the green tap for the OT specifically was drawing ~160mA according to the meter on my variac.
Is this normal? Just my multimeter loading it down?

It's normal for a PT, when energized, to consume power when unloaded. It's pretty typical for me to see 7-15 watts on the wall monitor, when I'm just doing the headphone trick or measuring voltages.

160mA at 120v is 19 watts--that's pretty high, but not frightening. Can you hear the PT humming?

[turbofeedus]

It's normal for a PT, when energized, to consume power when unloaded. It's pretty typical for me to see 7-15 watts on the wall monitor, when I'm just doing the headphone trick or measuring voltages.

160mA at 120v is 19 watts--that's pretty high, but not frightening. Can you hear the PT humming?

It was actually the OT that was the 160mA. When I put ~5V in the secondary yellow to black, measured the primary, the current meter on the variac measured 160mA.
The PT had some current draw as well, but it was less. 50-100mA depending on what I was testing.
I don't recall hearing humming, but I'll try it again and see.

EDIT: Just checked, the current draw in the OT was unchanged by the multimeter. I read 160mA from yellow to black, 90mA from gray to black, and 50mA from green to black.

[xtian]

I haven't noticed that before, with OTs, but sounds right.

[turbofeedus]

I haven't noticed that before, with OTs, but sounds right.

Hmm gotcha, well they seem ok, so they're probably not the fault that was in the original that prompted the fuse jumper.
I tested the filter caps, and they seemed ok.
Not sure, I guess I'll just start building the 5e6-a and test along the way.
Thanks for the info!

[xtian]

My advice: Use new filter caps! Old ones may measure OK with a multimeter or LCR meter, but under 400v the story is different.

[turbofeedus]

My advice: Use new filter caps! Old ones may measure OK with a multimeter or LCR meter, but under 400v the story is different.

Good call, there's plenty of room inside the chassis for axial caps as well.

[turbofeedus]

Quick question. I'm looking at the datasheet for 6L6, and under class AB pentode characteristics, there's two columns. Usually I would expect to see different plate/screen voltages, or cathode/fixed bias, but the two columns seem exactly the same except for current and load resistance. Is that the point? That if the tubes "see" 6600Ω vs 3800Ω impedance, with all other parameters staying the same, if will have the following effects on output wattage and plate/screen current?

[martin manning]

It was actually the OT that was the 160mA. When I put ~5V in the secondary yellow to black, measured the primary, the current meter on the variac measured 160mA.
The PT had some current draw as well, but it was less. 50-100mA depending on what I was testing.
I don't recall hearing humming, but I'll try it again and see.

EDIT: Just checked, the current draw in the OT was unchanged by the multimeter. I read 160mA from yellow to black, 90mA from gray to black, and 50mA from green to black.

Applying 5V 60Hz to the low impedance of the secondary is going to result in some significant current. I prefer to apply the test voltage to the primary (a-a) with a load connected across the secondary. You get current flowing in both the primary and secondary, and therefore a better answer for the voltage ratio. You can guess which leads are which for the first try, then connect the appropriate load to the appropriate leads. You can also dial up exactly 1V rms on the secondary and then read the voltage ratio directly (as rms voltage) on the primary.

Quick question. I'm looking at the datasheet for 6L6, and under class AB pentode characteristics, there's two columns. Usually I would expect to see different plate/screen voltages, or cathode/fixed bias, but the two columns seem exactly the same except for current and load resistance. Is that the point? That if the tubes "see" 6600Ω vs 3800Ω impedance, with all other parameters staying the same, if will have the following effects on output wattage and plate/screen current?

That's correct.

[turbofeedus]

You can also dial up exactly 1V rms on the secondary and then read the voltage ratio directly (as rms voltage) on the primary.

I believe that's what I did, except I used ~5VAC RMS. My fluke 179 does RMS AC readings, so I should be good there.
Thanks for confirming that bit about the datasheet.

If I punch in 6600Ω for load resistance and divide by my measured impedance ratios:
Yellow: 6600 / 1602.4 impedance ratio = 4.12Ω speaker
Gray: 6600 / 839.8 impedance ratio = 7.86Ω speaker
Green: 6600/ 470 impedance ratio = 14.04Ω speaker

[turbofeedus]

Another quick question, regarding the 5E6A schematic;
Interesting to see the OT CT and screens be connected to the same node on the power supply. Usually you'd see some kind of dropper resistor. Is it advisable to use some screen resistors? If the screens are at a higher voltage than the plates as indicated by the schematic, won't the screen draw excessive current?