Kathie was born, but ... (a bit long)

[KGW]

I got my first express running over the weekend. Her name is Kathie (my wife's name). She sounded pretty nice - clear and no hum or buzz. However, I may have a problem. I played her late Friday a bit and a bit the next morning. I turned her off , moved her across the room and hooked up my DMM to recheck the bias. The EL34s were drawing lots of current and I could not adjust it. The current was getting about 200mA and one of the tubes (V4) was getting red, so I shut her off, pulled the tubes, turned her back on and measured the voltages (all good). I tried swapping the EL34s (V4->V5, V5->V4) and the same thing happened. The current draw on both tubes was high, but V4 was higher and started turning red.

So, I powered down again and did some measuring and checking. I found a (I think) minor mistake: I had the B+2 connections wrong to the V4 and V5 such that the 1k0/5W resistors were bypassed. In other words, B+2 directly to pin 4 on V4 and V5. I fixed the mistake and tried again - same problem.

I started to suspect the tubes, so I pulled the EL34s from another amp and tried them in Kathie. These worked fine.

And now the big question: Did my mistake ruin the tubes? I rather doubt it since the 1k0/5W resistor was not in the original 50W Marshalls.

The tubes could have been defective. If not, I don't really want to burn out more tubes.

Any thoughts?

thanks

P.S. Dual bias, shielded filaments, 3 star grounds. Besides that, parts and layout are the same as Francesca (20uF TC Mallory filter caps on order. Spraque/Atoms in the mean time)

[KGW]

Here are more pictures. Note, I am having a cabinet made for me, so she will be naked for a while.

[KGW]

and some more...

[KGW]

and one more

[HiGain]

Looks nice

I'm not sure about your tubes, but...

How would you describe the sound, and could you please describe your shielding technique?

Thanks,
Jake

[KGW]

The sound is nice. It is clean to about 9:00 and starts breaking up after that. 12:00 to 1:00 provides plenty of headroom. My 1987 (DIY Marshall 50W) cleans up better with the guitar volume control, but the express has more gain. The harmonic feedback it awesome. The express is brighter than the 1987, but I could adjust the tone controls to get goot bass and mids.

I'll record and post some samples when I get the tube isuue resolved.

The shielding (I asume you mean the filaments) is from TUT1. PT filament wires to a terminal strip and separate shielded twisted pairs to each tube. The power tubes are grounded at the beginning of the cable (to the PSU ground buss). The PI and preamp tubes are grounded at the end of the cable (PI at PI ground buss and preamp tubes to the preamp ground buss). I made the cable using 20AWG twisted pair and the foil and braid from some coax. The ground wire is crimped to the foil and shield. Then heat shrink tubing around the whole thing.

I made my own cable for the V1 grids as well.

[rhinson]

you can't have 2 bias tail resistors-(you only need one)---they're in parallel to ground and in effect lower your overall resistance to where you don't have enough neg. voltage to have an appropriate bias range---with the 2 56k's you really only have a single 28k resistance---not high enough. just take them out and jumper those tabs and put in a single 47k. you'll be able to bias and you won't have a runaway tube like you did. rh

[KGW]

you can't have 2 bias tail resistors-(you only need one)---they're in parallel to ground and in effect lower your overall resistance to where you don't have enough neg. voltage to have an appropriate bias range---with the 2 56k's you really only have a single 28k resistance---not high enough. just take them out and jumper those tabs and put in a single 47k. you'll be able to bias and you won't have a runaway tube like you did. rh

Where did you hear that? I have a dual bias on my 1987 and it works fine.
The 2 bias pots DO influence each other, but you can match the bias of each tube easily.

[rhinson]

i'm not saying you can't/shouldn't do it----but depending on a bunch of things you may not be able to get away with it simply by using the same values over a second time for your second pot/resistor combo. just look at your schematic-----you have 2 20k pots and 2 47k resistors in parallel to ground as opposed to a single 20k pot and 47k ----this will reduce the neg. voltage available as compared to one of each (standard setup) because you just cut everything in half as your ground reference. now depending on the raw bias supply, your tubes, the plate voltage, the over all values and adjustment range, you may be able to do this just fine-----or not. you might have to use a 2 50k pots and 2 100k resistors to get the appropriate voltage needed. rh

[rhinson]

http://people.cornell.edu/pages/mt24/Amp/mm/bias.html

[KGW]

i'm not saying you can't/shouldn't do it----but depending on a bunch of things you may not be able to get away with it simply by using the same values over a second time for your second pot/resistor combo. just look at your schematic-----you have 2 20k pots and 2 47k resistors in parallel to ground as opposed to a single 20k pot and 47k ----this will reduce the neg. voltage available as compared to one of each (standard setup) because you just cut everything in half as your ground reference. now depending on the raw bias supply, your tubes, the plate voltage, the over all values and adjustment range, you may be able to do this just fine-----or not. you might have to use a 2 50k pots and 2 100k resistors to get the appropriate voltage needed. rh

You are completely correct about possibly adjusting the values and I did. 50k pots may be better, maybe not.

Note that the initial 220k resistor has been changed to 100k. Also, I was able to adjust the bias correctly initially. What happened in the 2 minutes when I was playing the amp and when I turned it off, hooked up the meter and turned it back on is the mystery.

[rhinson]

that being the case it sounds like a defective tube----i've had this happen numerous times with new tubes out of the box and had to send back to the distributor for replacements. unfortunately these days, even with the culling and testing most places say they do, it seems to happen all too often. if they're recent buys most places will be glad to send you a replacement. rh

[KGW]

that being the case it sounds like a defective tube----i've had this happen numerous times with new tubes out of the box and had to send back to the distributor for replacements. unfortunately these days, even with the culling and testing most places say they do, it seems to happen all too often. if they're recent buys most places will be glad to send you a replacement. rh

Thanks much.

They are under warranty. My friend has a tube tester and will test them for me tomorrow. Do you know of a source for cheap used unmatched EL34s for amp testing purposes?

cheers - kw