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I've started the initial powered this up with a lightbulb limiter and have some unhappy DC voltages... hoping someone can help give me a steer on where the problem may lie.

Limiter using 100W bulb is glowing faintly - which is possibly normal, but it's certainly soaking up some of the supply potential resulting in a reduced B+.

All voltages with JJ tubes installed...
Wall Supply: 167VAC... 230VAC is normal supply, meter is not true RMS.

B+ 232VDC which is 0.66 of 350VDC, due to the limiter in action.

V1, V2, V6 = ok

All measurements in DCV, (Expected voltages in parenthesis, being 0.66 of those in the build doc)
V3 - All voltages low.
P1 = 122 (145)
P2 = 17 (26)
P3 = 26 (49)
P6 = 124 (145)
P7 = 17 (41)
P8 = 26 (50)

V4 & V5 are high on Pin 3
P3 = 14 (8)

V7 & V8
P3 = 223 (227)
P4 = 217 (227)
P5 = 0 (Uncertain)
P6 = 0 (~144)
P8 = 12.6 (14)

Your help is greatly appreciated... thanks all

Edit I should add, I've gone through this pretty thoroughly already and cant seem to find where the fault may be... I can see where the Voltages should be but arent, but the components and connections test ok. Gonna keep at it and will update if I find the problem... thanks

_________________
Corey

Well I couldn't find a fault with the wiring, no matter how many times I've checked... So decided to switch from a load to a sacrificial speaker. What'dya know, it's working!

Sound check with a proper guitar & speaker... Still running on lower voltage with the limiter but sounds great so far!

Stephen I wonder if you could confirm whether those voltages look ok, they seem quite low on the PI, and I've already tried a different tube.
If you have no concerns I'll test it without the limiter, and re-measure things for peace of mind... Thanks again, Corey

_________________
Corey

nothing has blown up so I would go full voltage and remeasure.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Ok... Double checked everything, including tightening the grounds once again.
Will still need some help!

DC Voltages as follows: (Expected in parenthesis)
Wall 237VAC
B+ = 361VDC (350)
After R28 = 336
After R22 = 274
After R21 = 262

V1
P1 146 (155)
P3 0.9 (1)
P6 138 (155)
P8 0.9 (1)

V2
P1 140 (180)
P3 1 (1.5)
P4 263 (272)
P8 140 (180)

V3
P1 176 (220)
P2 29 (52)
P3 44 (75)
P6 175 (217)
P7 28 (62)
P9 44 (75)

Little bit of sleuthing here... I have transistor amplifier experience, but not really much with tube circuits. Hopefully have found the problems to start with, the plate resistors of V1-3 are all over 100K... R4 measures 114K, R6 108K and R7 133k! Can I assume replacing those resistors with tighter tolerance should bring the preamp tube voltages up? Anything else to check?

The EL84s look fine, but the 6V6s I'm not sure about, particularly pin 8. Both tubes measure the same.

6V6s
P3 348 (?)
P4 337 (?)
P8 41 (22)

I'm ordering some more 100ks today... thanks in advance

_________________
Corey

I can suggest you check all the resistor values feeding V3. It is off more than expected. Rest look ok.
Cathode voltage on 6v6 is too hogh if ot was in the ON position.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

You are measuring the resistors out of circuit, yes?

_________________
http://pdfelectronics.com

Thanks for your reply Stephen. The schematic I'm looking at is 'v6 plexi Dual power Tube with boost, first release 4 Oct 2012' by the way... I checked all resistor values... Most were within 10% tolerance, except those nasty 100k's already mentioned. Replaced all the 100k's in the end as I found a whole bunch of nice metal-oxide film resistors I had forgotten about in the cupboard! (love that! ) I also replaced R13 and R14 the 470k grid? resistors as I stumbled on some 1W carbon films while I was there too. But like I said, all the resistors now test within 10%.

So after all that, some improvement to V1 and V2, but no real change to the voltages on V3... and I've already tried another 12AX7, same deal.


The 6V6 cathodes are 21V once engaged, whew! So that one is solved... thanks for the suggestion!



Hey kurtlives... great reminder, thanks its often those simple mistakes!
Thanks guys really appreciate your time

_________________
Corey

What about the other resistors leading to the plates and the cathode reaistors. Are they close to spec?
Have you played the amp?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hey Stephen, sorry it wasnt clear but I've tested ALL the resistors in the entire circuit, I've replaced any not within 10% tolerance. Amp sounds fine, its just really annoying me... As an electronic tech by trade, not having the correct voltages inside, you must understand, lol!
If you're 100℅ sure on those voltages for this schematic, then I'm gonna solve this if its the last thing I do! I guess after spending so much time on it so far, I just want to double check those with you... Because the alternative is that I've still overlooked something, or there is some weird drift issue once the circuit is running... and that could lead to a failure down the track which Im hoping to avoid... Thanks, Corey

_________________
Corey

Are these voltages in the EL84 or 6V6 position? Just wondering.
Lets see if we can get some more input on voltages on v6....
For now, I would not worry. Dont obsess too much!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Take a look at this thread. Similar situation as yours. Maybe ajmackay has some feedback.

I should measure our v6 to confirm but it is out on loan.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Obsess? Who me?
https://www.youtube.com/watch?v=07So_lJQyqw


Cool thanks for the clues Stephen, so long as it's not bad for the expensive bits & sounds good... I'm happy... still a tube n00b & problem solving is all part of the fun for me. I have cabinets & tolex on the mind right now anyway, I'll update the thread later. Thanks again

_________________
Corey

i will remeasure our v6 when we get it back.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Tubes are 20% tolerance components, keep that in mind.

_________________
http://pdfelectronics.com

Yes - I have weird voltages when the 6v6 - EL84 Switch is switched... Basically i think that the voltages doubled
i.e
V4/5 P3 12 volts when engaged and 24 volts when not...
V6/7 P8 21 volts when engaged and 42 volts when not...

Stephen was going to double check when his 18 Watt Plexi was returned...

But, my amp still sounds great...

I might bring it up next weekend when I drop in to pick up my ODS....


Take care,
Andrew

Thanks for the input, Andrew... that's pretty much what I'm getting from the power tubes as well. It would be nice to get to the bottom of those PI tube voltages, although it looks like we're pretty similar there too

Just doing the final touches on the amp now, pics not far away!

_________________
Corey