DC heater supply issue

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Ten Over
Posts: 335
Joined: Mon May 25, 2015 5:27 pm
Location: Central California

Re: DC heater supply issue

Post by Ten Over »

Here is an updated schematic.

The 3.6K resistor is stood on end and mounted in holes that are 3mm apart. There is plenty of room to put that resistor flat like all of the rest. Makes me wonder if the original design had a zener diode instead of the 3.6K resistor.
You do not have the required permissions to view the files attached to this post.
User avatar
didit
Posts: 976
Joined: Thu Sep 25, 2008 3:37 pm
Location: Canada

Re: DC heater supply issue

Post by didit »

A schematic that sort of makes sense -- thanks. Zener subbed in is the simple and likely satisfactory answer. A small heat sink on the transistor too. Silicon is fairly cool handling transient current overload. Not so when running at or above design temperature.

Best .. Ian
Ten Over
Posts: 335
Joined: Mon May 25, 2015 5:27 pm
Location: Central California

Re: DC heater supply issue

Post by Ten Over »

Pictures of a take-out PCB:
You do not have the required permissions to view the files attached to this post.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: DC heater supply issue

Post by sonicmojo »

Wow, thanks for all the discussion and solution ideas! That's what mine looks like alright - model U. Currently I'm about 5000 miles from the amp at the Austin City Limits festival. I don't look forward to going under the board for the radial caps so I may try just the transistor and accessible resistors first since I can probably snip them from the top and splice. I've got caps and transistors ordered and they should get there before I do.
---------
Bryan
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: DC heater supply issue

Post by martin manning »

Looks like that section of the pcb was laid out for a two transistor regulator, and there are a couple more larger holes left empty. Possibly those were also intended for turrets. How old is the amp? If it's 1990's, the electrolytics are surely on their way out... might as well change them. I don't know how much rework the PCB will tolerate, so be careful.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: DC heater supply issue

Post by sonicmojo »

I replaced the 1000uF electrolytics and the transistor but it looks like one of the caps was the major culprit. I'm at 6.1 right now with a 56R in place of the 360R on the schematic. I've got some Zeners ordered to replace the 3.6K and will see if that gets closer to 6.3. I still need to address the lower heater voltage on the other tubes. It is at 5.7V right now across the board on 115VAC line voltage. I'm able to bias is at 35mA at about 500V plate voltage on the power tubes. The amp probably has the original Sovtek 5881s in it. It sounds pretty good as is but I'd like to get it right if possible. I understand the winding of these Sovtek transformers are not necessarily the best from a quality control standpoint....Is there anything other than the transformer that could be dragging down the AC heater voltage?
---------
Bryan
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: DC heater supply issue

Post by martin manning »

What's the voltage on the main hester winding with no load? Does it drop proportionally with each power tube addition? Remember 6.3 +/- 10% is ok, so you are not in bad shape really.

The 56R between the DC filament supply filter caps is pretty low. Maybe the voltage on that secondary winding is also low? Your line voltage is also on the low side at 115, which could explain what you are seeing. A larger capacitance on the first filter would help, but 6VDC output is fine.
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: DC heater supply issue

Post by Stevem »

The much stated acceptance range of heater voltage of 10% higher or lower makes for a huge difference in amplification in my testing!
Here's what I have found.

A 12AX7 tube testing out at 1250 GM or 100% ( a hard to find great tube meeting design spec's!) on my tester drops down to 80% with a 10% reduction in heater voltage!!!!
Now seing as most new tubes you by are lucky to come in testing out in the mid to low 80% range your doing yourself a big disservice by running only 6 vac on the heater since this will take a 85% testing tube down to 65% and you will not only really hear that, but feel it also as the tube reacts much slower to signal input way down there!

It might be a good way to fly for greasy blues , but not much else!!!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: DC heater supply issue

Post by sonicmojo »

martin manning wrote: Thu Oct 12, 2017 9:38 am What's the voltage on the main hester winding with no load? Does it drop proportionally with each power tube addition? Remember 6.3 +/- 10% is ok, so you are not in bad shape really.

The 56R between the DC filament supply filter caps is pretty low. Maybe the voltage on that secondary winding is also low? Your line voltage is also on the low side at 115, which could explain what you are seeing. A larger capacitance on the first filter would help, but 6VDC output is fine.
Before I tweaked the resistor, I was getting over 8VDC on V1 without a tube. I think I'll replace the diodes as well to see if that helps a little. I'll experiment with some different tubes to see how that affects AC heater voltages.
---------
Bryan
sluckey
Posts: 3062
Joined: Sun Jul 22, 2007 7:48 pm
Location: Mobile, AL
Contact:

Re: DC heater supply issue

Post by sluckey »

With that emitter follower circuit you need the tube plugged in for the voltage reading to be valid.

As a side note, that little DC circuit has an extra bonus feature that's not often mentioned... The output capacitance is equal to the Base capacitance times the transistor HFE. Actually a lot better output filtering than you would think.
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: DC heater supply issue

Post by sonicmojo »

Stevem wrote: Thu Oct 12, 2017 1:57 pm The much stated acceptance range of heater voltage of 10% higher or lower makes for a huge difference in amplification in my testing!
Here's what I have found.

A 12AX7 tube testing out at 1250 GM or 100% ( a hard to find great tube meeting design spec's!) on my tester drops down to 80% with a 10% reduction in heater voltage!!!!
Now seing as most new tubes you by are lucky to come in testing out in the mid to low 80% range your doing yourself a big disservice by running only 6 vac on the heater since this will take a 85% testing tube down to 65% and you will not only really hear that, but feel it also as the tube reacts much slower to signal input way down there!

It might be a good way to fly for greasy blues , but not much else!!!
I hooked up my Variac to test at exactly 120V and with a 100R resistor instead of the 360R, I am getting exactly 6.3VDC on V1. The amp is very quiet, no extra hum or buzz. The rest of the heaters are settling at 6.0VAC. Tubes unloaded I get 6.4VAC. Just as a a try, I implemented an artificial center tap on the AC heaters instead of one sided ground and it didn't change the voltages at all. I don't think anything short of a PT replacement will get me any higher with this amp. It works pretty well, not a lot of clean headroom but I think I'll just leave the amp for what it is.

Thanks for all the tips! I'll definitely be paying more attention to my heater voltages in the future.
---------
Bryan
Ten Over
Posts: 335
Joined: Mon May 25, 2015 5:27 pm
Location: Central California

Re: DC heater supply issue

Post by Ten Over »

sluckey wrote: Thu Oct 12, 2017 4:48 pm As a side note, that little DC circuit has an extra bonus feature that's not often mentioned... The output capacitance is equal to the Base capacitance times the transistor HFE. Actually a lot better output filtering than you would think.
The capacitance multiplier thing is what it is all about. The regulator part strikes me as just a way for the manufacturer to compensate for unknown winding outputs, especially with that one resistor on turrets. I documented one Mig-100 with a 3mV peak ripple on 6.1Vdc. Pretty impressive for two 1000uF caps.
User avatar
TUBEDUDE
Posts: 1656
Joined: Thu Jan 04, 2007 5:23 pm
Location: Mastersville

Re: DC heater supply issue

Post by TUBEDUDE »

I'd ditch the regulator/transistor and use the simple circuit Stevem posted.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: DC heater supply issue

Post by martin manning »

Stevem wrote: Thu Oct 12, 2017 1:57 pm The much stated acceptance range of heater voltage of 10% higher or lower makes for a huge difference in amplification in my testing!
Here's what I have found.

A 12AX7 tube testing out at 1250 GM or 100% ( a hard to find great tube meeting design spec's!) on my tester drops down to 80% with a 10% reduction in heater voltage!!!!
Now seing as most new tubes you by are lucky to come in testing out in the mid to low 80% range your doing yourself a big disservice by running only 6 vac on the heater since this will take a 85% testing tube down to 65% and you will not only really hear that, but feel it also as the tube reacts much slower to signal input way down there!

It might be a good way to fly for greasy blues , but not much else!!!
I agree that gm will go down as heater voltage is reduced, but ra will go up such that the amplification factor mu and the stage gain will be essentially unchanged at the test point. Note that in the plot below gm is reduced by only about 10% when the heater voltage is reduced 10%, and section 2's mu actually increased slightly. The tube is a new production 12AX7EH.

In an actual circuit with fixed Ra and Rk, where the operating point is allowed to shift, reducing the anode current by 10% at all Va and Vg (like running at 6V Vf) in a typical stage with 100k Ra and 1k5 Rk) causes a gain reduction of 4.3% or 0.38 dB. Not a big deal IMHO.
You do not have the required permissions to view the files attached to this post.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: DC heater supply issue

Post by martin manning »

TUBEDUDE wrote: Thu Oct 12, 2017 11:38 pm I'd ditch the regulator/transistor and use the simple circuit Stevem posted.
That circuit will work, but it will have much higher ripple, around 500mV. In fact Marshall's 4001 Studio 15 uses a FWB feeding a 1000uF cap, which results in 1.6V ripple. I have one of those amps and it is quiet, so it's not too important. Note there should be only one ground reference, as shown in the schematic snip below.
You do not have the required permissions to view the files attached to this post.
Post Reply