UA 610 Preamp - Transformer Substitute

[PRR]

> The power transformer is 500vdc at the secondary.

The plan you posted shows 275V at second node which suggests 350V at first node. Even unloaded, 500V seems awful high.

 

[dawsonaudio]

What I meant was the secondary transformer is rated for 500V with a 117 volt input. 

Without any tubes installed, the Voltage is around 385 at the first node and 380 at the second.  Shouldn't there be a a larger voltage drop after R29 on the LA2A power supply schematic?  I have a 4.7k/2 watt resistor installed as per the schematic.

 

[dawsonaudio]

Just for kicks, I opened up my LA2A I built some years ago and the voltage drops to 265 or 275vdc at the second node depending on the 117vac coming into the unit.  Discharged the caps, pulled the tubes, checked voltages again and the voltage is just like I'm getting on the 610 build. 

Shows how much I know about tube circuits.  I do know how to safely discharge caps, that's for sure. 

I just didn't want to 'fry' any tubes at startup with too high of a plate voltage.

I think I'm ready to put the tubes in, power things up and check the voltages at each plate resistor.

 

[Winston OBoogie]

Without any tubes installed, the Voltage is around 385 at the first node and 380 at the second.  Shouldn't there be a a larger voltage drop after R29 on the LA2A power supply schematic?  I have a 4.7k/2 watt resistor installed as per the schematic.

If there weren't any tubes installed, the voltage on either side of the 4k7 would be substantially the same value, as you discovered. 
With the tubes installed, the drop across that resistor can be calculated from ohm's law.  Equally, the current that flows in the tubes can be calculated from the drop across the resistor:  I = V/R.

I don't remember the exact current of a single channel of UA 610 but it'll be about 12mA so, we'll end up dropping about 50V.  The LA-2a all told draws a bit more current so more voltage will be dropped. 

So, to drop more voltage for the 610 we'd use a bigger value resistor.  Or, a 2nd resistor and cap after the 1st.
What value resistor?  Calculate this using Ohm's law


Not sure I understand your question regarding the input stage. Can you post the schematic snippet you're using?

 

[Winston OBoogie]

P.S.  i was happy to read you've mastered discharging the caps before poking around .  Please be careful and take care

 

[dawsonaudio]

So here are the voltages I've got at this point.  Did a quick bench test before I installed the board into it's resting place.  I currently only have the primary of the output transformer hooked up.  The input transformer is disconnected as well as the secondary of the output transformer.  Voltages are really high...  I've attached the schematic with my voltages inputted in 'hot' red.

C15 (82pf) was eliminated so the cathodes have no connection?
C8 (.002uf) was also eliminated so no connection there either?

 

[Winston OBoogie]

Yes, they're a bit high, looks like you could use a 2nd resistor and cap on that power supply if you have spare parts on hand.  Use another 4k7 and whatever cap you have for now.
Might have been a bit optimistic in remembering the whole unit as  12mA, looks like it's less.  However, I think there's something  off with the 2nd  stage 12AX7, that's a low bias.  Check your anode and cathode resistors, also the grid leak R and the valve itself if you have another you can pop in. 
The other stages look to be biasing up just fine

Edit:. Just noticed your cathode on stage 2 is indicating a fluctuating voltage, yes something not right there. 

 

[dawsonaudio]

Added a third resistor, only had 4k/2watt and cap and it drop another 10vdc.    Voltage coming into the first 40uf/450 cap is 381, 2nd 40uf/450 cap is 269 and third 40uf/450 cap is 358 and change.  So I'm assuming that if I wanted to drop it another 80volts, I'd drop in a 32k/2watt resistor?  Not sure if that is ok to do.

Just trying to solve one problem at a time if that is the right way to go about this.

Thanks for the help here.

Edit: That seems like too much of a voltage drop needed.  Would it be better to get a different transformer all together?  Maybe use a tube rectifier to drop the voltage?

 

[Winston OBoogie]

Added a third resistor

I thought there was only one resistor (a 4.7K)  initially.  Is there a schematic of your power supply?



So I'm assuming that if I wanted to drop it another 80volts, I'd drop in a 32k/2watt resistor?  Not sure if that is ok to do.



Well, I don't think your second stage is currently working correctly, a bias of .2V is low.  The voltage drop across the power supply resistors will increase when that 2nd stage is pulling more current.  Granted, not by a drastic amount,  but it should increase. 
I also don't know the current of your output stage as you left off the cathode voltage.  But dropping 10 volts over a 4k power supply resistor implies that your whole amp is only pulling 2.5mA  and the output stage alone should be pulling about twice that amount.


Edit: That seems like too much of a voltage drop needed.  Would it be better to get a different transformer all together?  Maybe use a tube rectifier to drop the voltage?

Well, it was a little on the high side to begin with.  You could put a shunt resistor on the end of the supply to pull more current and drop more voltage across the 4k resistors etc.  It's generally a good idea anyway to bleed off caps quicker.    Use ohm's law.  And don't neglect power dissipation of the resistor, a 1/2w won't do.

 

[dawsonaudio]

Here's my supply with the added 4.7k which is really a 4k and the additional capacitor.  So three 40uf caps and 2 resistors...Transformer is Allied 6K88VG TRANSFORMER; PRI:110-120V; 50/60HZ; 500VCT @40DCMA; 6.3V @ 2.0A; LEADS

 

[dawsonaudio]

I appreciate your help here.  I swapped out the 12ax7 and pin 8 voltage is stable.  Just for giggles, I rotated the gain (250k pot) fully clockwise and took some voltage readings.  Here are the new readings.  Red numbers are with gain knob fully counterclockwise.  Blue numbers are with gain know fully clockwise.

 

[Winston OBoogie]

OK thanks for the updated schems Nate.
I don't really understand why you're getting different voltages with the pot up versus down with no signal going in tbh.
But I see that with the pot up the current of the output stage is about 6.5mA which is where it needs to be.  And that the 2nd stage is now better.  And the B+ at 300v is about where I expected it was going to be with that extra 4k resistor.  300v is more than original but is fine in my opinion, tubes are very lenient with that stuff as long as you don't exceed dissipation which you aren't. 

Might be time to wire up the rest so you can listen and test further.  You need to figure out why the voltages go up and down with the pot.

Edit:. OK, looking again, I think there's possibly a wiring issue around the EQ stuff or feedback of the 12AY7 output valve.  If it doesn't work when you listen, maybe bypass the EQ stuff at first by taking the left side of R22 straight to R15.
Lemme know anyway

 

[dawsonaudio]

I'm not sure why I'm getting voltage differences either.  I haven't had a chance yet to run audio through it yet, but did take out the eq portion as you mentioned and the voltage still changes when adjusting the 250k pot.  This is with no signal coming in at the input transformer...isn't even connected.  Just to confirm the wiring on the pot is correct, pin 1 goes to ground, pin 2 to pin 3 of the 12ay7 and pin 3 of the pot connects to the side of the .2/c14 cap.  I've eliminated the lf and hf all together.

I'm going to go through my wiring again for the n'th time to see if I can locate my mess-up if I did.  Checked multiple times last night and everything looked good compared to the schematic drawn.

I'll keep looking.  Thanks for the help here.

Nate

 

[Winston OBoogie]

Just to confirm the wiring on the pot is correct, pin 1 goes to ground, pin 2 to pin 3 of the 12ay7 and pin 3 of the pot connects to the side of the .2/c14 cap.  I've eliminated the lf and hf all together.

Yep, that sounds correct. 
OK, get it wired for sound and see if you can find a clue.  Do you have any test gear, oscilloscope, meter etc?

 

[dawsonaudio]

No scope but maybe I need one. I do have a meter.  I will do some more testing later

 

[PRR]

Does the change of DC voltage happen abruptly at some gain knob setting? Suspect oscillation.

 

[dawsonaudio]

Nope, the voltage gradually increases/decreases when rotating and then comes to a stop when the knob stops.  I'm going to quadruple check tomorrow and reference the schematic again.  I've done it several times already, but I've got to be missing something, maybe a cold solder connection.  Maybe start from scratch again with all of the components.

 

[Winston OBoogie]

Hey Nate, starting from scratch is a lot of work. 
Have you wired up the output to listen for any clues?  See what happens if you unsolder the top of the pot to isolate the input and output sections.  You could post pics if you think another set of eyes would help.

 

[PRR]

If the DC varies slow, it is DC leakage. What DC is on all three ends of the gain pot at various settings? Clue: should always be "zero", a few mV, never >50mV.

 

[dawsonaudio]

Just rewired and reset all the components this morning.  Voltages are stable/better.  No voltage change with 250k pot adjustment.  Rearranged the layout a little bit so maybe that had something to do with it or maybe just a bad solder connection.  Here is my updated voltage readings.  Any changes necessary or tweaking before I mount the components in my box?  Won't have access at that point.

I'll go ahead and mount the board and components in the box this evening and post some images this evening or tomorrow. 

Thanks for the help here again.

Nate