Princeton Reverb Trouble

[RightLurker]

I'v just completed some work on a fairly beat BF PR - all new electrolytics (all the filter caps had been previously changed to 80mf - now they are back to the original 40mf), installation of an adjustable bias circuit, addition of 470 ohm 1 watt screen resistors, all new JJ tubes, and a 3 wire power cord. I have biased the power tubes.

The amp is a disaster. It sounds okay with the volume at 3 or below, but above that it puts out an ugly, harsh, brittle distorted tone, and up around 7 the low notes are followed by an ungodly fluctuating, kazoo-like wail. This isn't a sound that's on top of the note - it's a sound the amp is driven to make all on it's own after the guitar strings are muted.

Any ideas on what I'm dealing with here? Inadequate filtering? Parasitic oscillation? Any suggestions from those of you who know what you're doing (clearly I don') would be appreciated.

Thanks in advance.

RL

[RightLurker]

A correction to my initial post - the filter caps are 20mf, not 40mf. 20mf is the original specification. I should also mention that I've run the amp through two different speaker cabinets, and the problem was the same with both, so it's not the speaker.

RL

[tubeswell]

It sounds okay with the volume at 3 or below, but above that it puts out an ugly, harsh, brittle distorted tone, and up around 7 the low notes are followed by an ungodly fluctuating, kazoo-like wail. This isn't a sound that's on top of the note - it's a sound the amp is driven to make all on it's own after the guitar strings are muted.

I'm thinkin' microphonic tube(s) - did you try subbing/swapping the pre-amp tubes around?

[RightLurker]

Thanks, Tubeswell. I'll give it a try - although I'm thinking something so simple would be too good to be true. Here's hopin'.

RL

[selloutrr]

tubes first, then
tested continuity to make sure you have all the proper grounds and cables tracing to the proper destinations.
look for bad solder joints.
then test components you may have a bad pot, resistor, or cap. it should be a quick fix.

I might be wrong but I'm understanding it as the whole amp is sounding bad? not just the reverb?

test the OT
and reverb Transformer while your at it.

shouldn't take 15 min to go threw the amp and once you know what is working you'll know where not to look

[RightLurker]

The reverb (and trem) both work and sound fine - it's the amp as a whole. Thank you kindly for the advice - I'll see what I can find.

[RightLurker]

Well I'll just be darned. It was a bad tube in the V1 position. The tube in question was a brand new JJ ECC83. I swapped in a 12AX7 stamped "Westinghouse - Made In Russia", the problem went away and the amp sounds great. Just for grins I put in a different, brand new JJ, and the problem came back. I tried some other, old/used ECC83/12AX7s that I have, and they all work fine. I've had great luck with JJs, so this is a surprise to me.

I have a pair of brand new JJ 6V6s in the amp, which I just biased. One tube bounces around between 20.3 to 20.7ma at 416 plate volts, and the other bounces between 18.5 to 19.0ma, also showing 416 plate volts. That works out to 8.528 and 7.779 watts, respectively, if my math is right. I don't know how well matched those numbers indicate the tubes are, but the amp sounds great.

My thanks to both you guys (and everyone else here).

RL

[selloutrr]

glad to help

[Structo]

According to my calcs, 17ma is 60% and 20ma is 70%

That is using 12 watts as max.
The JJ's can probably do 14 watts without breaking a sweat.

Here is a nifty Excel program that Duncan Monro came up with.

Just enter your plate voltage and then the max power in watts, then click on the power chart and it will give low to high bias amounts.
Pretty slick.

[RightLurker]

Tom - that program looks like a fantastic tool. Unfortunately I can only look at the static image on this (Apple, non-Excel) computer, but tomorrow I'll download it on my office machine. Thanks for the help, everyone. This forum is great.

RL

[tubeswell]

@RL - Just for your edification/ejamuckation, hold the known microphonic tube firmly near the base between your thumb and forefinger, and put it next to your ear and give it a good flick with one of your other fingernails. It should sound quite jangly/loose compared to a specimen of the same type of tube that is known to be not microphonic. (This works better with 9-pin pre-amp tubes). Why did I suggest this? (just for kicks?) Its a kinda spit test for potentially dodgy tubes.

[iknowjohnny]

Tom, that doesn't take screen current into account, right? So i assume the 37ma i get at 475v/70% for el34's should equate to about 42ma on the meter. Thats what i've been using now.

[Structo]

No it doesn't take in account screen current.

I don't usually mention that especially to newbies since it will only add to the confusion and it can act as a safety margin for the bias.

[iknowjohnny]

thanks.

[RightLurker]

I took Tubeswell's advice and tapped a known good ECC83S and a bad tube of the same type with fingernail while holding the tube close to my ear. The bad tube sounds like a little crystal bell - it goes "rrriinnnggggggg". Sort of like tapping the rim of a crystal drinking glass with a fork. The good tube, on the other hand, just makes a very solid "tink" sound - no ringing whatsoever. I actually got stuck with at least two brand new JJ ECC83S tubes that are bad like this. I've used JJ ECC83S tubes in the past without an issue. Could it just these are just from a bad batch or bad run?

Anyway, very interesting. Thanks for the tip, Tubeswell!

RL