Help implementing Loop please Mr Glasman

[grumpy]

Hi Gary, or others who may know.
I really need some assurance/help regarding my D/lator type loop.
I have a separate node to supply B+ between PI and O/D nodes.
Q1: Is one node OK for the whole loop?
Q2: The voltage across the first 12ax7 stage of the loop has me worried, at the moment I have 315v at the plate and 18v on the cathode, this seems like a high voltage across the tube, is it correct?
Q3:I know Gary recommends at least 340v B+ which is 25v higher than mine, and I see there's 370v on the first stage in Gary's Rev a Loop but I don't want to adjust my supply until I know it's ok to do so. What should the cathode voltage be at V1 of the loop?
Q4: If I use a switch to bypass the loop, does it need to ground the input of the loop when it is bypassed?

Thanks in advance! I'd really appreciate advice from those with functioning loops built in to their amps, especially you Gary

John

[groovtubin]

Hi Gary, or others who may know.
I really need some assurance/help regarding my D/lator type loop.
I have a separate node to supply B+ between PI and O/D nodes.
Q1: Is one node OK for the whole loop?
Q2: The voltage across the first 12ax7 stage has me worried, at the moment I have 315v at the plate and 18v on the cathode, this seems like a high voltage across the tube, is it correct?
Q3:I know Gary recommends at least 340v B+ which is 25v higher than mine, and I see there's 370v on the first stage in Gary's Rev a Loop but I don't want to adjust my supply until I know it's ok to do so. What should the cathode voltage be?
Q4: If I use a switch to bypass the loop, does it need to ground the input of the loop when it is bypassed?

Thanks in advance! I'd relly appreciate advice from those with functioning loops built in to their amps, especially you Gary

John

I`ve used ONE node for LOTS of these amp w/D`ators, NO prob`s whatsoever!! I keep my NODES in series too btw... Only separate if B+ too low such as a Traynor YBA-4......... Q2, if that`s the CLEAN FIRST stage, something is WRONG, Q4, FIX the FIRST problem at stage ONE first.... Open plate, or bad tube...

[grumpy]

Thanks Groovtubin, but I'm asking about the first stage of the loop, not the whole amp.
I'm no amp guru, I just need to know some test/reference points for my own peace of mind. Gary has 370v on the equivalent plate in his Rev a loop schem , but what should the cathode voltage be? It's the voltage across the tube that concerns me most
This info would really help me sort this out for myself.

Cheers, Grumpy

[glasman]

Q1: Is one node OK for the whole loop?

The "real" deal used two supply points. I would suggest dropping a parallel string off of the screen voltage point (after the choke).

Q2: The voltage across the first 12ax7 stage has me worried, at the moment I have 315v at the plate and 18v on the cathode, this seems like a high voltage across the tube, is it correct?

Well once again, according to masurement data that I have seen from the d-lator, the cathode follower was much higher (+350v). You are probably reasonably safe at 315. The stage doesn't offer gain, only a buffering action

Q3:I know Gary recommends at least 340v B+ which is 25v higher than mine, and I see there's 370v on the first stage in Gary's Rev a Loop but I don't want to adjust my supply until I know it's ok to do so. What should the cathode voltage be?

The cathode voltages will range from about 16 to a little over 20 depending on the plate voltage. Best bet still is to have a separate string off of the screen position. Then you can adjust only that section for the best results and it will not effect the rest of the stages.

Q4: If I use a switch to bypass the loop, does it need to ground the input of the loop when it is bypassed?

Probably not necessary but always a good idea, that way you keep any extra garbage off the power supply.

Gary

[grumpy]

Thanks Gary, that helps a lot.
There you go, 370v at the plate, 20 at the cathode. That's 350v across the tube!
As I said, I'm no tube guru for sure, but I would have been convinced that those voltages exceed the specs for the tube. I thought 300v was the max? I probably don't know how to read spec sheets properly.

Thanks again Gary, cheers from Oz, Grumpy

[dogears]

Wrong....

The cathode follower was last in the string. The measurement I had was 257v on the cathode follower.

B+ > 15k dropping> 33uf cap >100k dropping> 33uf cap

Gary, you knew that!

[glasman]

Wrong....

The cathode follower was last in the string. The measurement I had was 257v on the cathode follower.

B+ > 15k dropping> 33uf cap >100k dropping> 33uf cap

Gary, you knew that!

Not on every one!!

[glasman]

Wrong....

The cathode follower was last in the string. The measurement I had was 257v on the cathode follower.

B+ > 15k dropping> 33uf cap >100k dropping> 33uf cap

Gary, you knew that!

Given that 257 volts on the last point. The node at 15K and 100K would be about 360 volts.

Gary

[dogears]

Really? How so?? The one in question was an incorrect hand drawn... no???

Wrong....

The cathode follower was last in the string. The measurement I had was 257v on the cathode follower.

B+ > 15k dropping> 33uf cap >100k dropping> 33uf cap

Gary, you knew that!

Not on every one!!

[glasman]

Really? How so?? The one in question was an incorrect hand drawn... no???

Wrong....

The cathode follower was last in the string. The measurement I had was 257v on the cathode follower.

B+ > 15k dropping> 33uf cap >100k dropping> 33uf cap

Gary, you knew that!

Not on every one!!

You are probably right, I stand (or in this case sit) corrected.

First tap recovery second tap cathode follower.

Gary

Gary

[grumpy]

Thanks Scott and Gary for sharing what you know. This has helped me a lot!
Cheers from rainy Oz, John