Can't find 150UFd 300V Ecap

[H Chandler]

I'm recapping an old Bedrock amp. It's got a pair of 150 300V axial electrolytic caps in series right after the standby switch and just before the choke.

The rectifier is full wave using two pairs of series connected 1N007's.

Would a pair of 220 uF caps be OK?

Thanks

[stokes]

The 220uf will be fine,a little more filtering may tighten up the bass some,and may not even be noticeable.

[Don Moose]

I don't suppose you can source an 82uF 600V part?

(two 150uF in series comes to 75uF -- caps in series add like resistors in parallel)

[stokes]

I hardly think going thru the hassle of finding an exact or closer match is worth the trouble.The little experience I have with Bedrock was playing thru one in a studio 15 or so years ago,super loud and very clean as I remember,so if you have 75uf or 82uf or 110uf as the first filter aint gonna make much difference.Use the 220uf's in series and forget it.

[H Chandler]

Thanks

The 220's is how I'm going. I'm pretty sure there'll be no issue with the rectifiers. Just wanted to get a sanity check here.

Like you say, it's a pretty clean amp. It's basically a Marshall 50 with extras like reverb, post PI MV and effects inserts.

I'm not sure if size matters but this thing has huge transformers. Gonna try some KT 66's in it.

[stokes]

Yeah,absolutely no issue whatsoever for the rectifiers,solid state rectifier can take a whole load more capacitance than a tube.I've even used 100uf with tube rects and had no problem.As for the KT66's I would stay away from the Sovteks.I had 2 sets of them go in a couple of weeks in one of the amps I built for one of my sons.I got some GEC's and they have been going strong for a couple of years.I havent tried the new Tung-Sol reissue KT66's but they are made by Sovtek,I believe,I wouldnt have a lot of faith in them either.

[H Chandler]

I'll probably get the Shugangs

I've heard good things about them. They're a lot cheaper than NOS, and I'm not sure kt66's are the right thing for this amp anyway.

I'm actually looking at the Valve Arts, as I've heard they're the same thing, hopefully with a bit of extra testing and QCing. I've heard good things about them though.

I haven't heard anything about the TS reissue kt66's. I've used the TS reissue 6V6's and had one die in about 6mos of heavy gigging. They sounded pretty good in a BF Deluxe though. I've also tried a few sets of 5881's and they've been OK so far.

Thanks

[stokes]

I am not a fan of any of the current production stuff,Russian,Chinese or the European.I used the Sovtek KT66's just to try them out and see if my "distrust" of the new stuff might change,it didnt.In that amp I would stick with the 6L6GC's,they will take a lot more punishment than a KT66.

[earache]

Bedrock Amp Crackling

Hi Chandler and All;

Some advice is needed, can you help?

I have a Bedrock amp that had a burned screen resistor on the back of one of the power tube sockets. I replaced it and now the volume fluctuates and it makes crackling noises. I've since replaced the EL-34's with a matched set of Svetlanas, I've replaced the filter caps with some I found on EBay, which instead of 150uF 330v axials I used 150uF 250V radials, and I've tried swapping in different 12AX7s with no luck. Any ideas?

Thanks in advance - Earache

[stokes]

The fuctuatung volume sounds like it could be a power supply problem,you said you had a burnt screen resistor,do you know what caused it to burn?It is usually caused by a tube going south.After you replaced the resistor,did you use the same tubes,before replacing them?If so,it is possible that you fried the resistor again.Check the voltages at the tube sockets pin and at the other side of the resistor on the B+ side.
As for the filter caps you used,are you sure the 500v rating you have with the 250v caps in series is enough,I dont know what the voltage is at that node,but I suspect it is probably around 500v's.The symptoms you are experiencing now,instable volume and crackling is most likely related to either the screen or the caps you replaced.I wouldnt advise using filter caps you "found on ebay" no telling where they've been or for how long.I'm not saying the caps are definately the cause of the problem at hand,but could become one down the line.Always best to use caps you can be sure are fresh and are rated for more volts than the amp can dish out.

[Chuck_Farlee]

I am not a fan of any of the current production stuff,Russian,Chinese or the European.I used the Sovtek KT66's just to try them out and see if my "distrust" of the new stuff might change,it didnt.In that amp I would stick with the 6L6GC's,they will take a lot more punishment than a KT66.

That Sovtek "KT-66" is nothing more than a regular Sovtek 6L6 in an ST-shaped enveloped.

AFAIK, the only current production KT-66 is the Chinese one. I believe that there are two plants producing it in China, but am not 100% sure.

Groove Tubes used to sell a KT-66HP, but it is no longer in production. It was a real KT-66 in terms of ratings, although most prefered the tone of the Chinese KT-66 to the Groove Tubes.

chuck

[earache]

...you said you had a burnt screen resistor,do you know what caused it to burn?It is usually caused by a tube going south.After you replaced the resistor,did you use the same tubes,before replacing them?If so,it is possible that you fried the resistor again."

I did try the same tubes again but the resistors couldn't have fried like before as the resistor was truly burnt in half. I don't know what caused the resistor failure to happen before.

"I wouldnt advise using filter caps you "found on ebay" no telling where they've been or for how long."

I got the caps from a reputable dealer who has an EBay store
http://jecapacitors.com/index2.html

They aren't the old wax caps that someone has salvaged and is selling as NOS

[stokes]

A bad resistor doesnt always look burnt.Also,sometimes the solder will melt before the resistor actually burns so check the solder joints as well,a loose connection here can cause the problems.I had exactly the same symptoms in an Ampeg someone bought to me,although it wasnt a heat issue,it was a bad solder joint on one of the screen resistors.Although the voltages looked good on both sides of the resistor,when I did the chopstick test,poking the resistor with the bad solder joint caused the noise.It could be just a case of not soldering the joint secure enough.Try resoldering the resistor you changed.The vibration when playing will cause the bad joint to be problematic.

[earache]

Thanks for your input, I'll try doing some troubleshooting this weekend with the amp open and turned on. I hope that poking around will turn up the noisy component

Earache

[stokes]

I would start with that resistor you replaced,it is the most probable cause,before poking anywhere else.