powe surge damage--?

[rfgordon]

I just finished a new amp build (kind of a cathode biased 5F6A). It was great. Last night I was working on tweaking the V1 cathode resistor/cap combo to get it just right--(it was REALLY windy here yesterday--lots of power lines down)--and the lights flickered off then on. Suddenly the amp has a very dark hum and if you engage the bright switch it shreiks horribly.

Any ideas to offer before I start tearing out parts?

[mhuss]

Does it have a valve or SS rectifiers? If the latter, check those.

--mark

[UR12]

Have you measured your voltages after your outage occured?

[rfgordon]

Turns out the push/pull pot for the gain was self-destructing. I replaced it with a regular 1M pot and used a toggle sw for the bright cap. Works fine. Rocks like a moogerfooger. The big noise problem was really just a high-gain circuit at work. Not really any noiseyer than my other gainy amps. I was so freaked out at themoment I saw problems that weren't really there.

I can report that the PRP audio series resistors are great. I also used 100ohm CCs on the grids of V1 and the PI for a wee bit extra warmth.

[David Root]

Good to hear the case is solved, as they say. Which PRP metal films are you using, the GP or the PR9372, and where do you get them? I've been using the GPs for two years but I hear the PR9372s are even better.

[rfgordon]

I use the PRP PR9372. I got the first batch by requesting a sample set from the Precision Resistive Products website (www.prpinc.com). I got one whole amp's worth of them. They seem to be working out well. Different character than regular MFs--not bright or spikey. I also used them in a harp amp I just built. That one's like a 5D4 with reverb--very low gain--so it's so quiet there's almost no noise at all till you dime the volume control, and then all you've got is some tube hiss.

[David Root]

That's so easy it never occurred to me! Just ask and they send samples? Unheard of these days unless you're a big corporation. Are you sure you didn't tell them you worked for Fender?

I shall try it myself.

[rfgordon]

A lot of companies will send you sample sets. I also got resistors from Reidon--metal film .65 watts. I used 'em but I didn't think they were as warm as CFs. The down side on Reidon is that they will only sell them to you in lots of 100 per value. They called me up, and when they heard how small my business is (+/- 10 amps a year) they lost all interest.

PRP, on the other hand, sells thru vendors, like www.soniccraft.com, and the PRP guy I spoke with was very encouraging to me. Wanted to know how they sounded, etc. Nice guy.

I haven't tried the sample thing yet with any cap companies, but I may. I recently worked on a Holland amp for a guy, and it had all the caps branded "Holland." I don't know how many you have to buy to get them to stick your name on them!! I reckon it's quite a few.

Has anyone tried the new Vitamin series caps from Mojo?

[David Root]

No, this is new to me, but interesting. I have a small stash of (and know how they sound) the original vintage '50s-'70s Sprague "Vitamin Q" paper in oil caps which Mojo is obviously trying to capitalize on. IF mojo's Vitamin T's are as good as those, or the other good mfrs of the era, like Westcap, Gudeman, Aerovox and the Euro and USSR equivalents, they will be a good deal as they are a bit lower in price than NOS.

I looked at the pics on mojo's site. They cleverly don't show the ends of the caps--the originals are glass and seal very well to the aluminum case. This is the main reason the originals are still good after 30 to 40 years in storage, they don't leak. Also no details of the construction at all.

At this point I'm skeptical. I would like more detail and particularly where they are manufactured. There are a lot of overpriced so-so "supercaps" out there IMHO, mostly plastic caps using exotic materials, tin, copper, silver, teflon etc.