5F10 FUN

JamesAM

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Glad you sorted this and got a working amp. It was a really great troubleshooting thread for the startup procedure and I definitely learned a lot!
 

screefer

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Glad you sorted this and got a working amp. It was a really great troubleshooting thread for the startup procedure and I definitely learned a lot!

Thanks
I learned tons as well!

I still have to go to bias school and hunt down a few gremlins, but I gotta say that the folks here are amazingly generous with their knowledge and, especially with me, their patience! :)
 

King Fan

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Good job, sir.

Forgive me for revisiting LBL details. I may have missed lots; I’m just catching up to this thread. Lots happened! I see my guess about the no-tubes startup not lighting up your 150W bulb echoed by smart people. But do I read right that you were measuring voltages on the LBL? Maybe not. If so, though, it makes sense they measure better on the 150… more current, less limiting. But also, recall voltages and sound on any limiter may be inaccurate — sometimes by a lot.
 

screefer

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Good job, sir.

Forgive me for revisiting LBL details. I may have missed lots; I’m just catching up to this thread. Lots happened! I see my guess about the no-tubes startup not lighting up your 150W bulb echoed by smart people. But do I read right that you were measuring voltages on the LBL? Maybe not. If so, though, it makes sense they measure better on the 150… more current, less limiting. But also, recall voltages and sound on any limiter may be inaccurate — sometimes by a lot.

Thanks KF.
You get a huge chunk of credit for my success.

I used the 150 watter for the last three start overs and it never came on even with the power tubes.
Yes it sounded crappy until I removed it.

My off LBL voltages on the B+s are 332,330, and 264.
My original target, from a post months ago, is the stock 305 so I will have to devise a strategy to achieve that without lowering the heater voltages too much. I have the rigging for a bucking transformer so we'll see.

Thanks again!
 

2L man

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Great that you found the problem yourself! For your future builds and for anyone else troubleshoots one valid "pro method" to isolate problems is to "short" signal path. Practically always when stage is cathode biased and there is zero/ground potential on Control Grid then it can be shorted to ground and most anything what exists on previous stages does not pass that shorted place. Technically if previous stage oscillates it can effect other stages thru power rail but it is very small what there can come if for example pre amp stage oscillates.

I tack solder a piece of wire or use two small clips and four inch of wire in between but it seems to be lost always I need it ;)

For transistor circuit troubleshooter it is two needles and 4" of wire which is used to short transistor Base to Emitter always when it looks like the Collector is shorted with Emitter. I have found dozens of shorted or leaking transistors using this needle trick because collector Coltage rise to its operative voltage when transistor is fine and does not rise if it is shorted or leak a lot.
 

screefer

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Today I will bias the power tubes.

I found this GREAT discussion...https://www.tdpri.com/threads/questions-about-5f11-biasing.1055193/ ... and
will follow the processes described therein.

I have a strange issue with my 6AT6 socket.
The ceramic/plastic part has come loose from the metal "carcase?' and can be deflected a 1/4".
These are cheap looking units. I got them from AES for like $1.25.
Yesterday I had an intermittent scratch coming out of the speaker and when I wiggled the 6AT6 tube the scratchiness increased. When I held the tube to the input side of the chassis the scratch went away.
This morning it is dead quiet no matter how much it is moved/wiggled.:confused:
I have a spare socket.

I also had some slight intermittent motorboating yesterday which I attributed to maybe the B+ 1 Cap being compromised because I fired up without the OT center tap attached on one of my Three startups.
That noise has also gone away.

I'm having fun and perversely grateful for my mistakes and issues as that makes learning easier for me.
Thanks again to you all!
 

screefer

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I biased my amp and here's what I have settled on for now.

I first used my 1 ohm resistors from cathode to ground.
V3 was 27.37 mv .002737 amps
V4 was 23.25 mv .002325 amps
My plate voltages are 314.

I plugged this into Rob's Calculator and got.
V3 PD= 8.7w. PD% 61.4
V4 PD= 7.3w. PD% 52.1

I then tried Rob's OT Resistance method.
Using mini-grabbers I measured voltage drop from OT CT red to brown pin 3 on V3 (3.97)
and blue on V4 (3.52).

I then powered off and after the Caps drained I measure OT resistance across those same points.
I got 155.3 on V3 and 162.3 on V4.

Dividing V3 VDrop by the 155.3 resistance I get 25.57 ma.
Dividing V4 Vdrop by the 162.3R I get 21.68ma.

Are the differences between the two methods negligible?

The amp sounds great and after 4 or 5 adjustments I ended up with the 10k piher pot pretty much in the middle.
 

NTC

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The difference in current calculations between the ot measurement method and the cathode resistor method is due to the cathode measurements including the the screen current. About 2ma each is about right. Personally, I think you could bias it a bit warmer (both over 60% dissipation) but if it sounds good, it is good.
 

screefer

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The difference in current calculations between the ot measurement method and the cathode resistor method is due to the cathode measurements including the the screen current. About 2ma each is about right. Personally, I think you could bias it a bit warmer (both over 60% dissipation) but if it sounds good, it is good.

Roger on the screen current.
Thanks for pointing this out. Now I own that bit of knowledge. Very cool:)

It does sound good but we’re still getting acquainted!
 




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