Master Brightness Caps in Non-HRM Amps

[David Root]

TM, V2 is a '65 RCA short gray plate 12AX7, plate voltages are V2A 204, V2B 205 @ wall voltage = 121VAC.

Lou, I did mean 270pF, oops! I had them in the HAD approved plate to cathode mode, but the hash was awful. Then Colossal told me he and others had good luck putting them from grid to cathode. Reasoning is that way you're cutting out the HF to ground before it gets amplified at the plate. I thought I noticed an improvement, but the hash is still there.

I just tried the amp again after diddling with the lead dress around V2 but no improvement noted.

BTW I have the OD entrance trimpot wiper at HAD spec 25.6K to ground, so I'm not putting too much signal to V2A.

[ToneMerc]

TM, V2 is a '65 RCA short gray plate 12AX7, plate voltages are V2A 204, V2B 205 @ wall voltage = 121VAC.

Lou, I did mean 270pF, oops! I had them in the HAD approved plate to cathode mode, but the hash was awful. Then Colossal told me he and others had good luck putting them from grid to cathode. Reasoning is that way you're cutting out the HF to ground before it gets amplified at the plate. I thought I noticed an improvement, but the hash is still there.

I just tried the amp again after diddling with the lead dress around V2 but no improvement noted.

A little warm , what's V1 voltage? For grins, try dropping it 20 volts and see if you hear difference.

TM

[aflynt]

I agree with TM. Lowering the voltage on V1 and 2 definitely softens things up.

I'd also look for a long plate for V2. I like a short plate in V1 and a long plate in V2. To my ears the new JJECC803S sounds great in V2 for a softer smoother sound.

-Aaron

[David Root]

Interesting! I tried to keep V1 and V2 plates as per Tony's layout, V1 around 200V (mine are 195 and 200) and V2 around 205V.

I do know that lowering V1 plates voltage on 2nd gen amps is very nice, I think Tony did that first, to about 180V or so, and it works nicely on mine. My 1st gen amp is even lower and also sounds good, although the OD is still a tad aggressive.

Conventional wisdom for tube choice on V1 and V2 is long plate on V1, to get those long plate harmonics, and short plate on V2, to minimize microphonics and tighten up the bass. I've done it the other way round and with long plates in both slots.

My 102 has a long plate Mullard in V1. I originally had a long plate Amperex in V2 and changed it to the short plate RCA in case that would help with the hash but it doesn't seem to.

[CHIP]

I've read numerous times to run those wires around the preamp tubes directly down from the board and onto the chassis floor.
What about metal films on the cathodes?

[David Root]

I generally do that with the plate and cathode wires, and get them side by side too where it can be done.

Metal films on V1 and V2 cathodes I don't think I've ever done. I generally use Piher or Iskra CFs there, there's no high voltage stress on them so they don't tend to drift with age. Did I miss something?

[ToneMerc]

Interesting! I tried to keep V1 and V2 plates as per Tony's layout, V1 around 200V (mine are 195 and 200) and V2 around 205V.

David, I not try to be 100% tethered to the preamp voltages on these layouts. I treat each one as an individual and tune it as needed.

TM

[talbany]

IMO...the 1st 2nd and some 3rd generation amps can tend to have a bit more of a raw or unrefined sound (especially in OD) Perhaps due to the 100k plate loads and the funky 3 resistor OD entrance network (Much higher knee) so lowering these to around 180 or so can help with the buzz factor also helps the amp feel a bit more organic..On the flip side if you like an aggressive top end with more bite you can always keep it around 200v or so.. Of coarse different type preamp tubes can also effect this to some degree..After you have done a few of these and have experimented around with the dropping string with these earlier amps you can then use pre amp voltages as a tuning point to get the amp to respond to your individual tastes like Mike mentions..I have seen voltages on V1 in the ODS very from 155 on up to 220..

Have fun with it!!

Tony

[David Root]

Thanx TM, I'll definitely try that.

I think I am also going to change out the PT to get to 465V on the plates. I have a spare of two I had made for my 183 build which will give me that 465V. It will slide right in, no mods. The current PT is a Mag. Cpts. Twin PT, it seems to be on the low side of spec. for a Twin.

I will also try out the cap across the V2 gridstoppers too.

[David Root]

Tony, thanx for the advice. I know the different generations seem to have succeedingly higher preamp plate voltages. You were the first to get my attention about lowering them, when I was building my 2nd gen.

If lowering V1 and V2 on my 102 will get rid of this HF hash I'll be very happy! It's splatty even when I dialed the entrance trimpot wiper to ground down to 14K.

What's your take on the plate voltages? I'm inclined to take TM and Aaron's suggestion to up them to your layout values.

[talbany]

Thanx TM, I'll definitely try that.

I think I am also going to change out the PT to get to 465V on the plates. I have a spare of two I had made for my 183 build which will give me that 465V. It will slide right in, no mods. The current PT is a Mag. Cpts. Twin PT, it seems to be on the low side of spec. for a Twin.

I will also try out the cap across the V2 gridstoppers too.

Dave
I just looked at the amp..are those TVAN's (V1 V2)the ones you got off the Bay?

Tony

[David Root]

Yes, what a steel trap memory you have! They are only on V1 though. Is there a problem with those that would cause trouble in V2? They are just out of spec on the high side at 8uF.

[talbany]

Yes, what a steel trap memory you have! They are only on V1 though. Is there a problem with those that would cause trouble in V2? They are just out of spec on the high side at 8uF.

Just wondering here..I picked up a bag of those as well and found them to not be a smooth cap.. (If they are NOS some could have dried up?). I didn't care for them..Not saying that this is your problem..Just letting you know my opinion of them
IMO..Harshness/Buzzy/Grainy/gritty..etc are for me a kind of broadly used terms that is really difficult to diagnose and cure (especially up in the harmonic spectrum) without actually hearing the amp..Even on the bench it can be a real challenge..
The build looks clean so I don't suspect any real oscillation issues (although it's still a possibility)
I wouldn't think the lower wattage grid resistors would cause it to be harsh..
I also wouldn't think the lower B+ plate voltages would cause it to be the culprit either..
The cap across the V1B grid resistor could actually make the problem worse I do't see how it could make the problem better..
Exceeding the 300v max PI rating can make an amp harsh (also check around 58v cathode)
Also what kind of Presence cap is that?
What I would is try to isolate the problem.. Bypass the loop from the amp.. Then is it harsh in clean mode w/PAB on
Is the clean harsh or is the OD harsh or both..
Can you slave the pre out into another power amp..See if you can narrow it down..
Generally the first thing I do with a harsh amp is go over every solder joint with a high powered magnifying glass..

I don't know if any of this helps you..really just thinking out loud

Good Luck

Tony

[David Root]

Thanx again Tony! I bought them expecting +/- 5uF or so, they were around 7.5 to 8uF so I pointed that out to the Ebay seller and he promptly refunded my money and delisted them from his site, so I can't blame him. He didn't want them back so I kept them.

They are new old stock, 1988, the clean sound is good though. It's only the OD that's harsh.
I will change them out to the same as what's on V2, these are old NOS e-caps I got from Steve at Apex Jr., have been using them for years without problems.

The 500pF cap across the V1B grid resistor is in your layout, so I assume it's authentic. If changing out those TVANs doesn't help I'll certainly look at it.

The PI plates are at 288V and 287V, the PI cathode is 56.5V.

The presence cap is actually two 1uF 50V radial caps as I didn't have any 2.2uF caps when I built it. I now have some Sprague 2uF TE caps which measure about 2.4uF, I should change those 1uFs out, especially as they are Black Gates which was all I could find on hand at the time.

Clean mode with PAB sounds OK.

I'll check over the solder joints too.

[talbany]

The 500pF cap across the V1B grid resistor is in your layout, so I assume it's authentic

Sure is.

It's only the OD that's harsh.

Clean mode with PAB sounds OK.

Great.Looking at the pic and voltages I don't see any clues as to why the OD is harsh.. We all know you can build an amp, good choice of components,and voltages look OK?..

Hope you find it..

Good Luck

T