Metropoulos Forum

Hey Guys,Have not been over here for a while.I built a 50 plexie a couple years ago and everything was great.Now i keep redplateing my power tubes.I cant chance another set so can someone give me a plan to figure why this is happening.
Before I try again I was going to change the screen resisters "s1.5" and anything eles on the power tubes pins.I did put a Lar?mar when I built it.Anyway I would appreciate any advice,Thanx Mikey

need some more info....

Type of tubes, are they the originals you put in?

Plate voltage (pin 3)
Bias setting (how and what value)?
Build spec?
What trannies?
Does it do it all the time or only when you crank to 10 or hit it with a boost pedal?

Great to hear from you Neil ,Ill get the info together.You helped me through the build on this a couple years ago.The trannys are the Mercuries.It was a straight up Metro Plexie build with a Lar/mar master.I have ti get some coffe in me and wake up.Its still dark here.Im thrilled you picked this up,I remember how nice it was to post a guestion and when I got up in the morning you had the answer for me.Be back shortly,Mikey

OK,Neil,I powered up standby on and pins 3 on the power tubes were in the 360"s.Tubes started to red out again and I shut it off.I think what happened was when I swithched from the original tubes that were proper bias the amp redplated,My guess is I took out something eles at that time.Im hoping that I didnt do any transformer damage.Do you think I should get some new screen resisters and any other resister following to the bias control,Ill wait to hear from you.Thanks Neil,Mikey

Hi
Suggest you take the power tubes out and check the voltages on the output tube pins and post them here. Makes ure that when you fire the amp up with output tubes that you have a load on it (I use a Hotplate set to load) or you could use aspeaker cab set at the correct impedence.

It is very unusual for redplating with no input, so yes another fault is highly likely, bias could be waaaaay off (like a bias cap failed or similar). Also measure the screen resistors for continuity (ie 1k).

Check these things first and psot back. A picture of that part of the amp would be helpful.

Hey Neil,Im having trouble posting pictures perhaps if need I email them to ya.
The reading on v4& v5 are the same Full on /no tubes
Pin 1=0
2=3.3 h
3=254
4=254
5=-44
6=460
7 =heater
8=0

Your voltages on the plates (pin 3) and screens (pin 4) are way down. Should be roughly the same so if pin 6 is 460v and you have a correct screen resistor it should only be a smidge lower.

Bias voltage on the grid is about right.

yes need some good pics, links will do

http://s660.photobucket.com/albums/uu32 ... t%20metro/" onclick="window.open(this.href);return false; Hope this helps,Could the low voltage be that the bias is dialed back,Thanks

ampmike wrote:Could the low voltage be that the bias is dialed back

Sorry to jump in for Neil, but no. The bias will have no effect without tubes installed. Are you sure those pin 3 & 4 voltages are correct? Does your meter have a fresh battery?

We need to find out why pin 3 and 4 are so low. Those voltages suggest that something is wrong with both screen resistors and the OT, which is very unlikely.

Just to rule things out, with the amp off and caps drained, measure the resistance across each screen resistor(big white ones on V4 & V5 sockets). They should be about 1k. Then measure resistance from V4 pin 3 to the OT center tap (turret with brown and black wires in the corner of the board by the bias diode) and do the same for V5 pin 3. Post those readings.

The following has nothing to do with the low voltage on pin 3 and 4, but...
You said you have a PPIMV but the 220k resistors are still on the board. Do the green and orange wires on pin 5 go to the PPIMV pots?

I've thought about the possible effects of leaving the 220k resistors connected and I don't think it will affect the bias or potential redplating. However, it may and I just haven't figured out why (likely). Regardless, I would remove those 220k resistors.

Ok,Yea,Ill get to checking that I just have a bit of a busy weekend,I may not post till Monday,Thank Guys,I cant tell you how much I appreciate the help.I can post more Pictures of the hole board if needed,Thanks Mikey

Yea,Just had a minute,Pins 3&4 are now 460 V,So thats good,Also resistance is correct on the screen resisters and the pin 3 to the corner of the board turret,{Black and Brown wire} is 38.Maybe I just have a few batches of bad tubes now.Also this amp need to be blown out with air.Ill do that and after we sort this thing out Im going to go through the lead dress.It was my first amp build and its far from a clean build.
Thanks Gents,Talk to Ya shortly,Mikey

Well,I have a set of decent el34"s.Should I remove the 220"s first and then try to fire up and Bias?,Mikey

Thanks Guys,I think we ore OK now.No more redplating.I resoldered all the power socket pins and began to shorten some leads and clean up a bit.The el34 are not matched so I would like to buy a decent set,Any suggestions?The mullard reissues were sounding real nice before they crapped out.These are SED"s but way off.I tried to set the bias arounf 38 mill.I do appreciete your help.Thanx Mikey

Hey Mikey! What is the voltage with tubes installed?

I think you might be right at the edge with your bias @ 38mv.