redplating 1959 w larmar

[Roe]

...
Roe...your original post makes absolutely no sense. Re-read it...there's a conflict in it -

two tubes redplates on the 480v setting. usually these are v4 and v5 but the redplating changes to v6 and v7 if I swap [the] sides [of v3's plate wires]. the redplating is always on the push side (not the pull side), the side that normally is connected to the 82k in the pi.

You state that "the redplating is ALWAYS on the PUSH side"...(i.e. the side driven by the 82K), however when you swap the tubes to different sides it switches to V6-V7, which is the 100K driven side. So how can the redplating ALWAYS be on the 82K driven side if the problem follows the tubes when you swap them to the 100K driven side?
...

above I tried to say that the redplating is always on the push side, this side is normally - but NOT necesarily - driven by the 82k. when swapping plate sides in the PI, the 100k side was redplating.

I'm expecting oscillation (or too much AC in the output stage) in this amp. different tubes all redplate. in some cases I've gotten 3-4 four output tubes to redplate.

edit: I dont think its AC. at least this amp has less AC under load than my 67 100w

[flemingmras]

No I think that the bias is too hot for the plate load and plate voltage. Try biasing colder...remember what I had stated before that some amps cannot be biased to 70%? This sounds to me like one of those cases. The plate load/plate voltage combination isn't wanting to warrant being biased at 70%.

I would bias colder to the point where the redplating stops, then monitor the bias current at idle and note how much it is at that point. From there on out that will be your maximum bias point.

Another thing you can try is to mismatch up a setting (i.e. run your 16 ohm cab on the 8 ohm tap). This should limit the plate current by doubling the plate load and may allow you to bias up to your 70% figure.

Now another question I have...if you have the PPIMV why are you still cranking the output section?

[raiken]

I'm expecting oscillation (or too much AC in the output stage) in this amp. different tubes all redplate. in some cases I've gotten 3-4 four output tubes to redplate.

If you suspect oscillation, disconnect your feedback wire and see if the problem goes away. If not, try at 10K - 33K resistor right on the pin of the input side of the PI. This will kill oscillation in most cases. A PI input resistor will also limit a bit of the distortion asymmetry in the PI.

Randall Aiken

[Roe]

Randall, the redplating does not go away w/o negative feedback.

Flemingras, it redplates just the same with cold bias

[raiken]

Randall, the redplating does not go away w/o negative feedback.

Oh, I missed that part. Sorry!

RA

[Roe]

No I think that the bias is too hot for the plate load and plate voltage. Try biasing colder...remember what I had stated before that some amps cannot be biased to 70%? This sounds to me like one of those cases. The plate load/plate voltage combination isn't wanting to warrant being biased at 70%.

I would bias colder to the point where the redplating stops, then monitor the bias current at idle and note how much it is at that point. From there on out that will be your maximum bias point.

Another thing you can try is to mismatch up a setting (i.e. run your 16 ohm cab on the 8 ohm tap). This should limit the plate current by doubling the plate load and may allow you to bias up to your 70% figure.

Now another question I have...if you have the PPIMV why are you still cranking the output section?

thanks for the suggestions.
First, cold bias doesn't help.
Secondly, running a 16ohm load from the 8 ohm secondary helps - no redplating
Thirdly, I always crank amps when testing. I want them to work running at 10 for at least a gig. I don't get redplating when I turn the MVs back to 7.

[Roe]

Randall, the redplating does not go away w/o negative feedback.

Oh, I missed that part. Sorry!

RA

no, thanks alot for all the suggestions. I've moved the OT secondaries quite a bit and the wires to the presence control also. but it doesn't help. I'm considering disconnecting the MV now

[raiken]

First, cold bias doesn't help.
Secondly, running a 16ohm load from the 8 ohm secondary helps - no redplating

I could be wrong, but I think these two facts, along with the fact that is doesn't redplate at idle (hence no likely oscillation), and it only redplates on one side, the same side when tubes are swapped (asymmetrical overdrive), and the fact that the loadlines clearly show operation above the max dissipation limits for the lower impedance and within the limits for the higher impedance, proves my point.

Another point - even if the amp were oscillating, as long as the output section is designed within proper dissipation limits, it can oscillate all day long without redplating - the amp doesn't know the difference between an oscillation and a signal, other than the fact that the oscillation is continuous and the signal is intermittent.

By the way, the RCA receiving tube manual recommends a 6.5K plate-to-plate load for the 6CA7/EL34 when running at 450V on the plates and 450V on the screens (that's for two tubes, it would be 3.25K load for four tubes).

Randall Aiken

[flemingmras]

First, cold bias doesn't help.
Secondly, running a 16ohm load from the 8 ohm secondary helps - no redplating

I could be wrong, but I think these two facts, along with the fact that is doesn't redplate at idle (hence no likely oscillation), and it only redplates on one side, the same side when tubes are swapped (asymmetrical overdrive), and the fact that the loadlines clearly show operation above the max dissipation limits for the lower impedance and within the limits for the higher impedance, proves my point.

Randall Aiken

Exactly what I was trying to say. I wonder what OT he's got in that thing. Could he need one with a higher plate load/impedance ratio for the plate voltage he's running? To me it seems that it's not one or the other (B+ or OT plate load), it's that particular combination of the two that he's running that clearly does not work. Or maybe that particular OT was wound with the incorrect turns ratio (not enough turns on the primary or vice versa). Would need a signal generator and a volt meter to find this out.

Mine runs 460V at the plate when biased to about 63-65%. But mine has a high gain preamp and as such I don't crank my output section since everything on mine comes from the preamp. I haven't noticed any redplating issues on mine with JJ E34Ls.

[Roe]

First, cold bias doesn't help.
Secondly, running a 16ohm load from the 8 ohm secondary helps - no redplating

I could be wrong, but I think these two facts, along with the fact that is doesn't redplate at idle (hence no likely oscillation), and it only redplates on one side, the same side when tubes are swapped (asymmetrical overdrive), and the fact that the loadlines clearly show operation above the max dissipation limits for the lower impedance and within the limits for the higher impedance, proves my point.

Another point - even if the amp were oscillating, as long as the output section is designed within proper dissipation limits, it can oscillate all day long without redplating - the amp doesn't know the difference between an oscillation and a signal, other than the fact that the oscillation is continuous and the signal is intermittent.

By the way, the RCA receiving tube manual recommends a 6.5K plate-to-plate load for the 6CA7/EL34 when running at 450V on the plates and 450V on the screens...

Randall Aiken

Randall, thanks again. perhaps better tubes will help. sofar I've used some junk el34s, winged Cs and two tung sol 6l6GC str - all redplate

[Roe]

First, cold bias doesn't help.
Secondly, running a 16ohm load from the 8 ohm secondary helps - no redplating

I could be wrong, but I think these two facts, along with the fact that is doesn't redplate at idle (hence no likely oscillation), and it only redplates on one side, the same side when tubes are swapped (asymmetrical overdrive), and the fact that the loadlines clearly show operation above the max dissipation limits for the lower impedance and within the limits for the higher impedance, proves my point.

Randall Aiken

Exactly what I was trying to say. I wonder what OT he's got in that thing. Could he need one with a higher plate load/impedance ratio for the plate voltage he's running? To me it seems that it's not one or the other (B+ or OT plate load), it's that particular combination of the two that he's running that clearly does not work.

Mine runs 460V at the plate when biased to about 63-65%. But mine has a high gain preamp and as such I don't crank my output section since everything on mine comes from the preamp. I haven't noticed any redplating issues on mine with JJ E34Ls.

I got the metro c1998 and 1203-80. it got 480v with a cold bias.

[raiken]

Randall, thanks again. perhaps better tubes will help. sofar I've used some junk el34s, winged Cs and two tung sol 6l6GC str - all redplate

What OT primary impedance are you running? Have you measured it? The procedure is explained here:

http://www.aikenamps.com/OutputTransformers.html" onclick="window.open(this.href);return false;

Or, for the short version:

The impedance ratio is the square of the turns ratio, which is also the square of the voltage ratio, as shown in the following equation:

Zp/Zs = (Np/Ns)2 = (Vp/Vs)2

If you put 1VAC across the secondary, and measure 20VAC across the primary, you have a turns ratio of 20:1, which corresponds to an impedance ratio of 400:1. This means that if you put an 8 ohm load across the secondary, you will get a reflected impedance of 3.2K ohms across the primary. If you put a 4 ohm load across the secondary, you will get a reflected load of 1.6K ohms.

For example, if you have a transformer designed for 4.3K : 8 ohms, you can apply a 1 volt AC signal across the secondary 8 ohm winding, and you should see 23.18VAC across the primary, which corresponds to a 23.18:1 voltage ratio or a 537.5:1 impedance ratio, which would reflect an 8 ohm load back as 4.3K.

For your 1.7K transformer, if you put a 1V AC signal on the 16 ohm secondary, you should get: 1V * sqrt(1.7K/16) = 10.31VAC across the primary.

RA

[flemingmras]

Randall...on that RCA data sheet, that would be for two 6CA7/EL34s correct? So 1/2 that for 4?

[raiken]

Randall...on that RCA data sheet, that would be for two 6CA7/EL34s correct? So 1/2 that for 4?

Yes, I edited it to say: "that's for two tubes, it would be 3.25K load for four tubes". You must have read it before I added that part.

RA

[Roe]

Randall, thanks again. perhaps better tubes will help. sofar I've used some junk el34s, winged Cs and two tung sol 6l6GC str - all redplate

What OT primary impedance are you running? Have you measured it? The procedure is explained here:

http://www.aikenamps.com/OutputTransformers.html" onclick="window.open(this.href);return false;

Or, for the short version:

The impedance ratio is the square of the turns ratio, which is also the square of the voltage ratio, as shown in the following equation:

Zp/Zs = (Np/Ns)2 = (Vp/Vs)2

If you put 1VAC across the secondary, and measure 20VAC across the primary, you have a turns ratio of 20:1, which corresponds to an impedance ratio of 400:1. This means that if you put an 8 ohm load across the secondary, you will get a reflected impedance of 3.2K ohms across the primary. If you put a 4 ohm load across the secondary, you will get a reflected load of 1.6K ohms.

For example, if you have a transformer designed for 4.3K : 8 ohms, you can apply a 1 volt AC signal across the secondary 8 ohm winding, and you should see 23.18VAC across the primary, which corresponds to a 23.18:1 voltage ratio or a 537.5:1 impedance ratio, which would reflect an 8 ohm load back as 4.3K.

For your 1.7K transformer, if you put a 1V AC signal on the 16 ohm secondary, you should get: 1V * sqrt(1.7K/16) = 10.31VAC across the primary.

RA

Randall,

the OT should be 1k7. can I use 9-12v AC or is that too high?
I'm really abusing a pair of RTF el34s and one pair of new gold lion kt88s now and can get absolutely no redplating on the kt88s even with hot bias and punching riffs for 15minuttes. one RTF started to redplate after 15minutes with 65% bias.

b+ went down to 473v with hotter bias
my mass150 have never been this hot before - thats for sure.