Rev B ST35 (with Tube Rectifier) Help

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Rev B ST35 (with Tube Rectifier) Help

Postby veg1515 » Wed Nov 23, 2011 10:50 pm

Ok first let me say I am king procrastinator for waiting this long to get amp finished

I really want to get this done and start enjoting it so here goes

I had some problems before and everyone was a big help in getting me this far

I had a bad power tranny I am using Neds for this

Then got that worked out

now
when I hook up the amp and turn it on it has very low speaker output , sounds good though

Here are some pics

http://s194.photobucket.com/albums/z54/ ... 20Rev%20B/

I know a few have helped me with this before

What do you think I should start with?

I had a bad power tranny but did not check my out put trannys do you think they could be suspect?

here are my measurements

J1
1. 1M
2. none
3. 1M

J2.
1. none
2. none
3.1M

J3
1.none
2.none
3.1 ohm (when connected)

J4.
1. none
2.none
3. 1ohm (when connected )

J5
1. none
2. none
3. 1M

J6

not fully tested yet


V1
1. 148.5
2
3 1.9
4 3.3
5 3.3
6 147
7
8 1.86
9 3.3

V2.
1. 172
2. 148.5
3. 147.8
4. 3.3
5. 3.3
6. 172
7. 147
8. 146
9. 3.3

V3. V4 V5 and V6 all tested like this
1.
2.
3. 12.4
4. 3.3
5. 3.3

6.
7. 335
8.
9. 340


Thanks again for all your help

I stopped on this from being a bit frustrated some hopefully I can tacle this problem now I know it will be rewarding when I do
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Postby Shannon Parks » Thu Nov 24, 2011 5:36 am

Your low output is completely explained by what I see in your voltage measurements for V1 & V2. It looks like your voltages in V1 and V2 have the wrong tubes in their sockets. You don't have the 12AX7 and 12AU7 swapped, do you? Or are you subbing tubes? Try some replacements. V1 shows it is running over twice the current which changes the plate voltage level which screws up V2 since they are DC coupled. V2 is practically shutoff with just 30V across the tube.

Shannon
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Postby veg1515 » Thu Nov 24, 2011 8:30 am

Thanks for the help

I swapped the tubes they are now the opposite of what the Rev B board says they should go

here is what they are now

V1 is a 12AU7
V2 is a 12AX7

here are my new measurements

V1
1. 116
2. 0
3. 780MV
4. 3.3
5. 3.3
6. 116
7. 0
8. 780MV
9. 3.3



V2.

1. 213
2 116
3 116
4 3.3
5. 3.3
6. 213
7. 115
8. 118
9. 3.3



so these seem like they are close

I got a little more volume out of the amp,

The amp seems like at full volume its only has 1/3rd of the out put it should have

What do you think I should try?

Thanks
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Postby EWBrown » Thu Nov 24, 2011 10:34 am

V1 (the front tube) should be 12AX7 / ECC83, the rear middle tube should be 12AU7 / ECC82.

First thing, make sure that the tubes are OK, I've run into bad "new" ones in the past.

Secondly, check and verify with a DVM all the resistor values surrounding V1 and V2, I've built a variety of all revisions of these amps, and I still sometimes get the wrong resistor in the wrong place. (I'm partially color blind) Check the soldering too, for any "cold solder" joints, or poor "flow".

With V1 and V2 being direct coupled from V1's plates to V2's grids, any resistor value errors will throw the input driver and phase splitter voltages off, and that would result in low output volume, or unexpectedly high distortion.

The output EL84 / 6BQ5 tubes are DC-isolated from the driver circuitry, by the interstage coupling caps, and their v0ltages look pretty close to expected normal, the Cathodes (pin 3) are between 12-15VDC, that it is dependent on the plate voltage, and current being pulled through the EL84s (nominally, 35 mA), or 0.35VDC on each of the individual test points.

Update 11/25/11:

Another thing to check, did you pre-set the two NFB trimpots (R30 and R31) to approx 19K (for 8 ohms) or 27K (for 16 ohms) before testing the amp ?

If the trimpots are set to "zero" (factory default setting) or a low resistance valye, then the NFB is going to be excessive, and that will seriously reduce the amp's output, and would affect the 12AX7's cathode voltage, making it too low, which in turn, would cause it to draw excessive current, which would also lower the 12AU7 grid voltage, and throw off all the voltage measurements surrounding these two tubes.


HTH

/ed B
Last edited by EWBrown on Fri Nov 25, 2011 9:02 am, edited 5 times in total.
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Postby dcgillespie » Fri Nov 25, 2011 6:15 am

A couple of good clues you can use to help guide you -- and would help us as well:

1. Is the problem identical in both channels? If so, the only things that could cause that are making the same wiring error in both channels, or the power supply, which is common to both channels. If the problem is not identical in both channels, then you are chasing down two separate problems.

2. You say the output is low, but this could be due to either a loss of gain, a loss of power, or both. It would be very helpful to know which symptoms we are dealing with. If the amplifier will attain its normal output capability with low distortion -- but just takes an extreme amount of input signal to attain it, that is one thing. That kind of problem will always reside in the earlier stages or the power supplied to them. On the other hand, if the amplifier starts to run out of power and distorts well before normal output maximums are expected to be produce, then that is quite another. Those kinds of problems will almost always rest in the output stage, or its connection to the load. Earlier stages can cause this symptom as well, but I would start with the output stage first and then work backwards if this is the case.

Having the best and most detailed descriptions of the symptoms always helps to better zero in on different problems as they crop up. Let us know!

Dave
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Postby 20to20 » Fri Nov 25, 2011 7:58 am

edit
Last edited by 20to20 on Fri Nov 25, 2011 10:25 pm, edited 1 time in total.
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Postby EWBrown » Fri Nov 25, 2011 9:01 am

I presume that the filament voltage readings of 3.3V(AC) are correct, those voltages would be on either side of the filaments' resistive "center tap" and would add up to 6.6VAC, which is within the operating voltage specifications of all of the tubes.

/ed B
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Postby veg1515 » Tue Dec 20, 2011 11:00 pm

Ok so sorry it took me so long to post

here is the latest

I measured all the resistors most were good

some were off a little

I found two that were way off (I think they wrec 330K instead of 1.3K)

R3,R6 - I purchased some expensive resitors Kiwame or something along those lines (I think its factory erroe because the lines showed a 1.3K resistor)
I ordered the recomended ones from mouser along with all the the others that were a little off (have not replaced these , only will if needed)

I started by replacing the 2 and the results were night and day . I put in a new Ecc83s at same time and fixed a few cold solder joints

It still seems one of the channels has a little more volume than the other but could be in my head .

Next thing I found was a bad ecc83S tube -ordered that and put it in


The only major problem remaing is now I have some hum that I did not have before (maybe I had it but could not hear due to low volume ) Seems that the hum is only really apparant when I am on a source that has no signal

I do remember saying to myself before man this thing is quiet , so I think this is new.


Where should I start on the hum?

The last amp I built had hum and I put the trannys on cork to seperate them from the chassis .

Other notes I had a few cold solder joints I dr'd up , so that might have helped as well. I pulled old solder , scraped area and used flux to resolder . I am not sure of any other method to try for cold solder joints


So close to getting this done

Thanks for all the help!
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Postby veg1515 » Sat Sep 15, 2012 7:35 am

can you help with the above post ?

still trying to get the hum worked out

thanks
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Postby rmyauck » Sat Sep 15, 2012 12:34 pm

Is it present at the speakers or just the transformers as you talk about insulating them?

If it's from the speakers try moving wires around with a wooden stick or chopstick.

Also if from the speakers is it present in both?

Check RCA's , output transformers, and PCB board are properly grounded.

Randy
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Postby veg1515 » Sat Sep 15, 2012 12:47 pm

I will try that

Thanks for the advice !
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Postby rmyauck » Sat Sep 15, 2012 2:50 pm

Also loosen and tighten screws at terminals on PCB board. Maybe a bad connection there.

Also watch for ground loops.
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Postby veg1515 » Mon Sep 24, 2012 7:48 am

I was working a tube rectified version I think I migh eliminate that and see my results

anyone know where I can get a copy of the rev B st35 manual?

Thanks
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Postby Shannon Parks » Mon Sep 24, 2012 8:55 am

veg1515 wrote:I was working a tube rectified version I think I migh eliminate that and see my results

anyone know where I can get a copy of the rev B st35 manual?

Thanks


I'll email you one tonight.

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Postby veg1515 » Mon Sep 24, 2012 7:58 pm

Thanks !
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