Questions about rehabbing an old PP 6L6 mono amp with phono pre

[Fran604g]

The 5U4 has less voltage drop, so the B+ will be high. Something else to be aware of though, a 5U4 wants 3 amps of heater supply where a 5Y3 wants 2. Your transformer will not appreciate this.

I was thinking about that, I'm going see if a buddy of mine has a good 5Y3 I can scam off him.

At least now I have a couple ideas of what I'm doing. More or less...mostly less.

Thanks for all the help you guys have been. I'll chime in once I get the 5Y3.

Best,
Fran

 

[Fran604g]

Hey guys, I have a question:

What if this amp had been used with a ceramic phono cartridge? They needed high impedance, and values were apparently specified by manufacturers

Would that account for the 3.3M resistor on the 1st grid of V1?

 

[BinaryMike]

I suspect it had more to do with not overloading the phototube and keeping the size of the input coupling cap small.

 

[gadget73]

Ceramic cart would go with high impedance. Not sure what the output impedance of a photo tube is, but it probably also wants an extremely light load. If you're not planning to use it with a ceramic cartridge, no reason that grid to ground resistor can't be 470K or so and all the rest of the caps and resistors done away with.

 

[Fran604g]

Ceramic cart would go with high impedance. Not sure what the output impedance of a photo tube is, but it probably also wants an extremely light load. If you're not planning to use it with a ceramic cartridge, no reason that grid to ground resistor can't be 470K or so and all the rest of the caps and resistors done away with.

What specifically do you mean by "all the rest of the caps and resistors"? I removed all the front stuff associated with the mic and photo tube inputs. I'll be replacing the 3.3M with 10K as was earlier suggested, and the 330K resistor has already been replaced by 160K, also earlier suggested.

The 1M resistor was out of wack, too, at 130K. Crazy since it was a new carbon comp. I'm pre checking everything from now on.

I should've known better.

I'm hoping the 10K grid stopper will help eliminate at least SOME of the hum issue. I'll know in a couple of days when my parts get here. I'm pulling out the 5U4, too.

I'm doing a lot of reading, and becoming a little better acquainted with what's going on with the design. I have to say that this amp will probably sound like crap comparatively speaking - like you all have alluded to without saying it, thanks for being polite.

I'm having a blast, though. We'll get it working...maybe then I'll get serious about making it sound better.

As for the components, it sure doesn't have much in common with any of the simpler PP HiFi amp schematics I've studied. Or guitar amps, for that matter. Lol

 

[gadget73]

The other caps and resistors around that input stage. C19, R8, R5, R1, R7. Some of that is probably there to stand in for an EQ system for the phono cartridge and to maintain the extremely high impedance input that the ceramic cart and the photo tube likely wanted.

 

[Fran604g]

The other caps and resistors around that input stage. C19, R8, R5, R1, R7. Some of that is probably there to stand in for an EQ system for the phono cartridge and to maintain the extremely high impedance input that the ceramic cart and the photo tube likely wanted.

I like the sound of this idea. I'm thinking I'll do this.

Thanks!

 

[Fran604g]

Another question:

The phono pot goes to the 1M resistor (R5), which connects to C19, and R8, and both of them to R7. So can I eliminate the pot along with R5? I'm not sure how I run the 470K to ground, and have it still also be part of the phono pot from the input.

As it is now, the wiper from the pot goes to one end of R5, and the other end goes to a common lug with one end of R7. The other end of R7 goes to the grid. At the common lug attachment of R5 and R7 is also one end of C19 and R8. The other ends of them go to ground. If I eliminate R5, I no longer have the pot in the circuit.

 

[Fran604g]

I'm curious to hear what Tom Bavis might have to say. I hope he chimes in on the topic. He had said there was one stage too many

There is a bass cutoff in the negative feedback network - C9 - that should be removed (jumpered out). If you are keeping the tone controls, there's one gain stage too many (two too many if you eliminate them).

 

[Fran604g]

Hey guys,

I replaced the 5Y3 with a new Sovtek, replaced bad .001 cap (C19) put a 160K at R7, and replaced R8 (330K) with 10K, and fired her up.

She works with much less hum, and the record source is louder than the hum. There's still an appreciable amount of background hum, which might be expected considering the terrible wiring routes?

I'll check voltages later.

Fran

 

[Fran604g]

Also, for the record: jumpering C9 increased the hum.

 

[Fran604g]

Here's my YouTube short video of it in progress.

 

[Fran604g]

Hi guys,

I hate to keep piling up posts one on top of the other, but here's an updated schematic with the current values in the front-end.

I'm thinking I'd like to take the Phono Pot out of the circuit. The gain from it has to be "full" to provide an adequate volume level to overcome the hum, and hear the turntable. What might you guys suggest?

Both tone controls seem to be functioning as they should.

Hum increases with volume, and changes pitch with the tone controls.

Here's an updated schematic with any new values, and using a 5Y3.

The voltages as measured today, were measured with all controls fully attenuated, hum pot fully attenuated, turntable plugged into input.

Should I have set the heater voltage at exactly 6.0VAC before testing the voltages? I didn't think of that until after I tested everything, so they all measured at 6.6VAC.

Thanks,
Fran

Mains = 122VAC

B+
C13 - 10.4mVAC 24.3VDC
C14 - 2.0mVAC 277.5VDC
C15 - 74mVAC 335.4VDC
C16 - 5.1VAC 405.3VDC

5Y3
2 - 7.3VAC
4 - 354VAC
6 - 355VAC
8 - 5.2VAC


12AX7-1
1 - 31.8mVAC 209.4VDC
3 - 1.9mVAC 1.76VDC
4/5 - 3.2mVAC 0VDC
6 - 10mVAC 206.5VDC
8 - 1.9mVAC 1.78VDC
9 - 6.6VAC 0VDC


12AX7-2
1 - 48mVAC 251VDC
4/5 - 2.2mVAC 0VDC
3 - 29mVAC 86.3VDC
6 - 28.7mVAC 204.5VDC
8 - 2.0mVAC 1.95VDC
9 - 6.6VAC 0VDC

6L6-1
1 - 2.2mVAC 0VDC
2 - 1.25mVAC 0VDC
3 - 5.2VAC 400VDC
4 - 73mVAC 334.5VDC
7 - 6.6VAC 0VDC
8 - 10.6mVAC 24.3VDC

6L6-2
1 - 2.2mVAC 0VDC
2 - 7mVAC 0VDC
3 - 5.2VAC 400VDC
4 - 2.4mVAC 400VDC
7 - 6.6VAC 0VDC
8 - 10.6mVAC 24.3VDC

 

[BinaryMike]

Are you still using a magnetic phono cartridge? If so, then it seems appropriate to consider making a proper magnetic phono preamp input section. Perhaps someone else can suggest a useful example with 78 RPM equalization.

 

[Fran604g]

Hey Mike, actually what I "wish" to use is a Philco M-22 record changer from a Philco television/radio/record player console (in an earlier post I thought it was a Zenith), and I'm pretty sure it's a crystal cart (there seems to be some debate whether it might have actually been ceramic). There's so little info out there at the Antique Radio forum, but I did find a couple of service manuals for the unit online, and downloaded them. The cartridge appears to be a model 76-4649, but I don't know its output...yet.

I have my work cut out for me before I decide conclusively whether I'll use this turntable/cart combo, or go easy on myself and use the Miracord 10H. Right now, I need to concentrate on tracking down the source of the hum being injected into the circuit, I think.

The variable resistance in the front for the phono, what do you think that's all about? I assume it was there for matching the output of the different crystal/ceramic/magnetic carts?

Best,
Fran

 

[gadget73]

If you're running a crystal cartridge, the grid resistor on that tube probably needs to be a couple megohm like it was originally. Those things work into extremely high impedance.

The voltage levels are pretty different. Typical crystal or ceramic cartridge output is around 1 volt, typical magnetic cartridge output is around 5 millivolts. Its very much not apples to apples. Speaking of apples though, an ipod would have comparable signal level to a ceramic or crystal cartridge.

 

[Fran604g]

If you're running a crystal cartridge, the grid resistor on that tube probably needs to be a couple megohm like it was originally. Those things work into extremely high impedance.

The voltage levels are pretty different. Typical crystal or ceramic cartridge output is around 1 volt, typical magnetic cartridge output is around 5 millivolts. Its very much not apples to apples. Speaking of apples though, an ipod would have comparable signal level to a ceramic or crystal cartridge.

I was looking at some of the old RCA tube manual phono pres, and I'm half tempted to play around with maybe a switched circuit for either ceramic/xtal, and magnetic. I'm not sure I can cram any more under the chassis though.

I'm going through the M-22 turntable today, and if I can get it to function properly, I'll stick to my original plan. As it is, the pot metal cammed trunnion that lifts the motor idler pulley has done what pot metal likes to do, and expanded on the shaft it should slide up and down on to provide the 3 different speed positions. If I can manage to get it off the shaft and ream it so it once again moves freely, I'm confident the turntable will work, after replacing all the rubber parts that have calcified.

The cartridge shows high resistance, and the dual stylii look okay, so I'm hoping it might still actually function.

If not, then I'll keep the pre as is to use with the Miracord. As it is now, that setup works pretty well, except for the hum. I'm going to take a closer look at the tone control circuit to see if that's where the hum is being introduced. Maybe I got C8 too hot when I soldered it onto the pots. It has really short leads to make it fit into the space it's in.

I've thought of using my old Android, too. Just for fun. I really need to solve the hum issue first, though.

Best,
Fran

 

[trobbins]

Fran, I don't think you have done a hum test where input 12AX7 pin 2 is directly grounded to the 0V end of R9/C3. If you have noticeable hum for that setup, then do the same thing for pin 7, and then similarly for the next 12AX7 pin 7, and then shorting R23, and then pulling that 12AX7 out.

Hum can easily be picked up by wires and parts outside the valves, especially for circuits with pots that are some distance away.

If you still get hum, but its not associated with the output stage, then another check is to use a 6V battery to power the 12AX7 heaters (or at least the input 12AX7) to confirm whether that is not a major contributor.

There is no need to maintain heater voltage exactly at 6.3Vac, any voltage between 6.0 and 6.6V is well within tolerance and should have no consequences.

 

[Fran604g]

Sounds like a plan, thanks! I'll do that in the morning, and report back.

 

[Fran604g]

There are 2 distinctly separate hums:

#1 is in the "background", and is to my ears of lower pitch relative to #2 hum (probably the crappy wiring scheme). Doesn't change in pitch or loudness with any of the controls.

#2 is not noticeable until the volume is increased, and to my ears has a slightly higher pitch than #1. It increases with the volume control, and the pitch is changed by the tone controls. The phono control has no effect on the pitch, and only a very slight effect on the loudness - nearly undetectable to my ears.

The hum tests performed as you instructed:

V1 pin 2 to R9/C3 0V ground #2 hum is greatly attenuated (relatively speaking), but is still pretty loud, increases with the volume control, and the pitch changes with the tone controls. #1 hum unchanged.
V1 pin 7 to R9/C3 0V ground #2 hum is gone, #1 hum unchanged.

V2 pin 7 to C9 0V ground, #2 hum is gone. #1 hum unchanged.
V2 pin 7 to C9 0V ground with R23 jumpered, #2 hum gone. #1 hum unchanged.
V2 removed, #2 hum gone, #1 hum unchanged.

So does this tell me that #2 hum is being introduced at V1, pin 2?

And #1 hum is likely the crappy wiring layout (which I figured would be a problem)? For what this amp is, I can live with this slight #1 hum. It's reminiscent of many restored vintage electronics I've encountered.

Thanks,
Fran