1987X reissue issues......

[syscokid]

I found some old voltage readings I took in a span of 2 years: 120vac at the wall...455vdc at pin 3. 123vac/wall...465vdc/pin 3. Not sure how scientific this is...

Looking at your pics, It seems the amp has the proper Resistor values (for EL34's) from the PI circuit thru the Bias circuit. What value is the dual pot for the Master Volume?

[TheDirewolf]

It's a dual 250K linear. I bought the Lar/Mar kit from Valvestorm. Did I mention that Robert is awesome?!?
Here is some more background info on the tranny's. When I first installed them in the reissue prior to the board swap I was getting high B+ readings then too but they were in the 470 range. The PT also ran cool as a cucumber unless pushed hard and with higher PPIMV settings. Tonight however, it actually got hot. Not so much you couldn't touch it but hotter than it's ever been at the attenuated levels I had the PPIMV set at.

On a positive note, the amp sounds really good. The note bloom is nice. There's a sweet squish and squeak to the notes when dug in to and the sustain is the best it's ever been. It's either right as rain or it's fixing to die.

The only thing I have noticed about the PPIMV is my red wires are on the pot that the black wires are supposed to be on and vise versa according to the kit instructions on the cd that George sent me when I ordered the Zero Loss Loop. Should I switch them? Seems to be working fine.

I really do appreciate the help, btw. Thank you for your time and input.

[TheDirewolf]

So I bought new batteries for my meter just to rule that out and I'm still getting the same voltages with the preamp tubes installed and power tubes removed. I checked the neutral lug of the IEC socket and I'm seeing 155vac. How is this even possible? Or is that normal? I have to admit fellas, I've built a lot of things in my life, houses, hot rods, even guitars, but I have never been so bumfuzzled by something in my life. If anyone has any insight or suggestions I'm all ears.

[Roe]

Measure the following AC voltages:

-wall voltage/PT primary
- PT secondaries

Also measure DC at rectifier

[neikeel]

Make sure your input is grounded on the amp properly and the wall socket similarly.

[danman]

Do as Neikeel suggested and report back with the readings. If you keep measuring without the power tubes installed the voltages will be quite a bit higher. Take the measurements with all tubes installed and properly biased.

[TheDirewolf]

Ok. I installed all tubes and biased it to 30mA. PT secondary's are reading 363vac. PT primary at the power switch (no voltage switch was wired so I used the black 120V wire) is 43vac Rectifier + side is reading 486vdc as is pin 3 of V4&5. Pin 5 of each power tube is reading -46vdc. Now for the interesting part. My house was built in 1966 so it has been wired with the good ol' 2wire. I have no real ground. If I insert my ground probe in the ground socket of the outlet and read the Live side of the plug I get 43vac and the Neutral side reads 78vac. If you insert one meter probe in to the Neutral side and the other in to the Live side you get 121vac. I spoke with the electrician that does all of my work and he said that is typical of a 2 wire house. However, 155vac on the Neutral lug of my amp is stumping him as well.

[danman]

I really can't say why you are experiencing those strange AC readings. Plate voltage does seem a bit high for this particular transformer but if the amp sounds and performs well then you should be fine. I did notice in your pics that you fused your white(neutral) wire as is shown in the Metroamp instructions. This is how Marshall did it years ago but common practice now is to always fuse the black (hot side) for safety reasons. If a fault were to occur with the fuse on the nuetral side, it may not blow in time to protect the amp and yourself.

[TheDirewolf]

Ahhhh I see. So fuse the Line side instead of the Neutral? That makes more sense. I'm wondering if I have a PT that is bad or on its way out?

The amp sounds good for a new born. I'm going to swap the bright channel coupling cap as its a little on the noisy side. I'm hearing a little high end harshness but I think that's mostly the Svetlana's. My 2x12 cab with Creambacks is well broken in at this point in life.

I can't thank you guys enough for taking the time to hash this out with me. I'm truly grateful. Thanks guys!

[danman]

Play the amp for a few days and watch for any signs of redplating on the tubes and excessive heat from the PT. I do remember in the past that quite a few members mentioned that the Classictone PT's ran a little warm when the amp was pushed hard. The manufacturer stated that this was normal and as long as it didn't become too hot to touch then it would be fine. Maybe you just happen to get one that produces a slightly higher voltage than most.

[TheDirewolf]

I certainly will, Dan! I've been itching to play this thing.

Thanks again for the help everyone.

Just to be clear, the PT common and pilot light black wire will tie to the Neutral lug of my IEC socket bypassing the Mains Fuse and the Live wire to the power switch will then be attached to the Mains Fuse? Basically inserting the fuse between the Live lug of the IEC and the power switch?

[danman]

Correct...just swap the black and white around. The black should be the fused and switched line and white will tie to the PT's common (blue wire). It looks as if the IEC receptacle is wired properly with the white wire going to the wider (neutral) spade which would be at the end of the cord. Enjoy the amp and let us know how everything turns out!

[TheDirewolf]

Luckily there was a big "N" under that lug and an "L" under the other so even I couldn't screw that one up!

THANK YOU once again to everyone for the help! If the amp is ok I'll be installing the Zero Loss Loop next. Hope it goes well and I'll try and get some clips made soon. Not that anyone hasn't ever heard a Metro 50 watt before.

THANK YOU THANK YOU THANK YOU!!!!!

[TheDirewolf]

I played the amp all day yesterday without a single problem. I discovered one of the EHX EL34's I bought must have been bad or just couldn't take the voltages. I swapped some other EHX EL34's I had and even with the bias pot turned all the way clockwise it still was a tad high. I currently have a 56K resistor in line with the bias pot. Maybe bump up to a 64K?

I also installed the Zero Loss Effects Loop with no problem. I had to tie in between the 10K and 8K2 to get within range for the Loop B+. Turns out by doing so it dropped all my pin1 preamp voltages to the specs in the instruction chart. I didn't change the 8K2 to a lower value so the loop drawing current "starved' the preamp I guess? The only one that wasn't in range was v1 pin1. It was actually about 20vdc low. My question is how do I raise just the v1 pin1 voltage? Increase plate resistor or drop one of the two 10K resistors?

With the lower voltages and the lower filtering I was evoking a really nice Dutch Treat. REALLY nice actually. Had the squish and squeak that you hear on VH1. However it wouldn't clean up all the way with the volume like I need it to. Other than fine tuning and tweaking to my needs and possibly buying another PT, just because I can't handle knowing something isn't exactly "right" with it, I believe I'm just about done with this build.

Thanks for looking and I appreciate any and all help. Thanks guys!

[danman]

Raising the 56k up to 68k will help to bring the current draw down on that set of tubes. Most pairs will bias up normally but occasionally you will get a set that requires tweaking the bias resistor to get them in range. You may find that the next pair you buy will bias up to cold with the 68k resistor and you will need to go back to the stock value.

If you are happy with the tone then I wouldn't worry too much about a few volts difference on the preamp tubes. You can adjust the resistor values to tweak the voltages if you want to experiment. Raising or lowering the dropping resistor values (the 8k2 and 10k) will effect the whole preamp. You also have a couple of 10k decoupling resistors located near the preamp filter cap. Changing the value of the one located at the top of the filter cap will change the voltage to v1 only. I usually play with the value of the two dropping resistors on new builds to fine tune the sound. More voltage to the preamp will result in a slightly stiffer feeling preamp with more clarity. Lowering the voltages will yield a softer, more browner sounding preamp. The changes are very slight but you will usually be able to hear and feel he difference when making the changes.