RCA 41b Preamp

[lassoharp]

dmp,

Can you define the noise you're getting in a little detail?  Hum vs Hiss, plus freqs of.  I would imagine 120Hz hum is very low with the filter banks you have.



I assume the HK supply and your supply were equal distance away from unit?

How about PS feed cabling in one vs the other?  Any easy way to test?

 

[dmp]

I didn't measure the noise, but it is very typical of the noise from a grounding issue. I'll look at this again this weekend and hopefully figure it out.

I used different cabling in the test, but the connections were the same (4 wires, B+, gnd, Htr+, HTR-)

 

[emrr]

Make sure the heater taps on the pre don't have a ground connection of their own?

 

[dmp]

Make sure the heater taps on the pre don't have a ground connection of their own?

They do not.

Yes, I was thinking there's no ground reference drawn on the IP-17, yet I use it as well with no problems.  It must be there, undrawn. 

Mine does not. I opened it up and checked with a multimeter. Does yours have a mod to tie the heaters to the ground?

Can you define the noise you're getting in a little detail?  Hum vs Hiss, plus freqs of.  I would imagine 120Hz hum is very low with the filter banks you have.

I did a careful A/B comparison of the two PSUs again (IP17 and GR).  I discovered that there is a barely perceptible noise using both PSUs. With the GR, it is a low freq ground hum, while with the IP17 it is a higher freq buzz (probably due to the floating heaters).  Both are imperceptible with a hot mic. So... I think I've gone down the rabbit hole a bit and should just bring it over to the studio with the GR PSU before obsessing over it more. Maybe a RMAA measurement to quantify this.
In the GR PSU, the grounding looks correct. Maybe the heaters would need to be elevated to be 100% quiet?

 

[emrr]

It's been awhile, so I don't remember clearly regarding the IP-17.  I haven't added any CT, and don't remember what measurements I've ever taken.  I seem to recall adding artificial CT externally and finding no practical difference.  In practice it does operate quietly with several racks of 40-50dB preamps. 

 

[emrr]

You ever take more measurements?

 

[dmp]

I brought it to the studio to try out and it had a serious hum and was unusable. And that was after I thought I had the hum problem resolved.
I'm really struggling with this guy... ready to throw in the towel

 

[emrr]

I'd suspect the other 1mfd filter caps, but you say current looks correct.  Different tubes?  Overall orientation?  Tube sockets fully clean?  Sometimes just pulling and reinserting (or jiggling) tubes can get better contact.  Easy way to try DC filament? 

 

[emrr]

I was helping troubleshoot one of these remotely, and went over all my info.

There are at least 6 sub-models of 41-B, and the final 4205-D/E versions are the only ones I've seen a schematic for. I've had a 600755G3 and several 4205-D/E in hand to check out, the rest is from pictures. Known similarities or differences listed.

drawing 500206G1

77 tubes
2 jacks, no gain divider
RT-221 input
RT-222 output/choke
CP121 cap bank: 0.015/1/1/1


drawing 500206G2

77 tubes
2 jacks, no gain divider
RT-221 input
RT-222 output/choke
CP121 cap bank: 0.015/1/1/1


drawing 600755G2

77 tubes
3 jacks, gain divider
RT-221 input
RT-222 output/choke
CP121 cap bank: 0.015/1/1/1


drawing 600755G3

77 tubes
3 jacks, gain divider
RT-221 input
RT-222 output/choke
no R6/C6 treble boost, R10
CP121 cap bank: 0.015/1/1/1



4205-D/E manual copyright date is 1935

77's or 1603's. I have so far only seen two 4205-E's marked for 1603 tubes, all other D's and E's marked for 77's.
2 jacks
RT-376 input transformer update
CP-21 cap bank replaced with 66168-5: 10/1/1/2 mfd
R3 / 200K no gain divider on D, tapped resistor for -10 gain division on E
R10 / 70K on D, 120K on E

Changing the cap bank from 0.015 to 10 mfd probably erases a treble boost there, hence the addition of the R6/C6 boost.

on the 'E' I've got LCR meter data from two units at 120Hz:
input 250 : 30K
4.1H : 370H

output/choke 13K : 500 and 250H@3mA, 7K7R
75H : 2H7 and 174H 35KΩ

 

[emrr]

Replacing the old caps in the can. The tubular electrolytics for the cathode bypass get replaced
The square black ones are probably not electrolytic and I didn't replace them.

Several recent observations. 

After looking at many many amps of this age that lived in radio service for decades, it is normal for ESR to be very high.  A new cap may have ESR of an ohm or so, with the old one being 50Ω or more.  The piece may still sound fine, but would probably sound better with new caps.  There are those that don't want anything touched, which leads to quandaries.    Comparing against NOS equivalent era caps, the NOS will be much much lower, but still higher than a modern cap.    No one can say what the original ESR would have been when new, so not much point in thinking a lot about it.

This paper is interesting:

https://www.illinoiscapacitor.com/pdf/Papers/impendance_dissipation_factor_ESR.pdf

An important observation is the Fr parameter. Fr is the self-resonant frequency. Defined as the frequency where Xl and Xc are equal.

At this frequency the impedance is equal to the ESR.
Below self-resonance the Xc component is dominant and the capacitor behaves like a capacitor. Above the self-resonant frequency the inductive component is dominant and the capacitor behaves more like an inductor.

I just went through another late 1930's RCA piece.    ESR on the coupling cap was almost 80Ω.  The capacitance value of 0.5mfd  looked fine at 120Hz and 1kHz.    It's use caused a large treble rolloff compared to a new cap, no difference in lows.  As much as I wanted to keep using the groovy old paper in oil film cap, it was past the expiration date.  This is the only time I've ever noted this behavior, but then, how many 80 year old amps that are unmodified do any of us ever get to restore?

In this case the amp was on-air daily for 50 years, and then sat in an abandoned unconditioned space for 22 more years. 

 

[rackmonkey]

No one can say what the original ESR would have been when new, so not much point in thinking a lot about it.

You can calculate ESR if you know Tan[δ). Modern cap data sheets typically specify this. A quick browse through my old cap catalogs from the 30s - 70s tells me this is a relatively recent development. You don’t really see this figure specified until the 70s. At least I can’t find anything from earlier with that level of detail. Too bad.

Amazing that the caps worked at all after 22 years in the extremes. Where was the unit stored (geographically)?

 

[emrr]

.

Amazing that the caps worked at all after 22 years in the extremes. Where was the unit stored (geographically)?

A humid  place with temp swings from -10F to 105F. 

Get this:  All 10 electrolytics still measure ok and fired up without letting the smoke out. 

Of course I replaced them on principle.  When a 25/475 is 2” in diameter and 6” long, it may have some brute force qualities that resist the effects of age. 

 

[dmp]

Several recent observations. 

After looking at many many amps of this age that lived in radio service for decades, it is normal for ESR to be very high.  A new cap may have ESR of an ohm or so, with the old one being 50Ω or more.  The piece may still sound fine, but would probably sound better with new caps.  There are those that don't want anything touched, which leads to quandaries.    Comparing against NOS equivalent era caps, the NOS will be much much lower, but still higher than a modern cap.    No one can say what the original ESR would have been when new, so not much point in thinking a lot about it.

Interesting stuff. Maybe that was what caused the problem I was seeing. I never figured it out. I don't have it anymore either.

 

[DaveP]

Unfortunately, I have just seen this post.

For what its worth, if that had been mine I would have completely re-wired it with tinned wire, put in an RCA grounding system to the input socket, changed all the caps and replaced all the resistors with metal film.  Then I would have made a custom Power supply with DC heaters,  Then it might have had a chance in a studio with modern noise expectations.

This is not a criticsm DMP, I understand the reluctance to mess with such an old piece.

DaveP

 

[emrr]

That’s all total heresy Dave!  : ). The vanished pictures would have helped. 

Dmp did replace the electrolytics I believe, no small feat here since they are embedded in a multi-section cap box.

One mainly needs to follow the install protocols of the time, or adapt them. 

Perfectly capable of meeting modern noise expectations.  I just had someone tell me his 41-B’s are quieter than his various Telefunken preamps.

The wiring is more than fine, I dare say better than many living people will do, unless they spent years working for Telco. 

The resistors are all multi-chambered wire wound.  I'll add that while in many cases I've found improvement replacing V1 related carbon resistors with metal film, returns diminish to an insignificant level past that stage, with V1 self noise then dominant. 

Unfortunately, I have just seen this post.

For what its worth, if that had been mine I would have completely re-wired it with tinned wire, put in an RCA grounding system to the input socket, changed all the caps and replaced all the resistors with metal film.  Then I would have made a custom Power supply with DC heaters,  Then it might have had a chance in a studio with modern noise expectations.

This is not a criticsm DMP, I understand the reluctance to mess with such an old piece.

DaveP

 

[DaveP]

That’s all total heresy Dave!  : )

Yes, I know it is   I've had a non-conformist upbringing!

If the resistors are WW then they would have had a reprieve ;D

Best
DaveP