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Old 9th June 2009, 04:04 AM   #811
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Join Date: Oct 2004
Location: Adelaide South Oz
One page back autoteck1 suggested trying 6973 for the outputs.

I bought a quad of Electroharmonix 6973 to try. On the weekend I sat down to look at putting these in the BH. 1st problem they need a complete rewire to the tube socket. 2nd problem, their gm is half that of EL84 so I don't think they will suit the BH that well.

Accordingly, the EH6973 have been put on my shelf, they will suit a little guitar amp.

So not recommended for a Baby Huey.

I had the feedback settings up and down all over the place. What suited some music did'nt cut it for others. I'm running 6K8 + 17K8 + 6K8 now. (might as well be 18K for that feedback set resistor).

Cheers,
Ian
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Old 9th June 2009, 06:12 AM   #812
adamus is offline adamus  United Kingdom
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what wattage do the resistors need to be?
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Old 9th June 2009, 06:16 AM   #813
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I used 2W Metal Film for the 6K8, mainly for voltage handling rather than power handling and I used a Philips MRS25 (0.6W 350V rated) for the 17K8 cross connect. I'd just use 2W metal films for all of them - that way you are not tempting fate (the unkind Bas...d).
Cheers,
Ian
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Old 9th June 2009, 06:21 AM   #814
adamus is offline adamus  United Kingdom
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ian, apologies i should have explained more carefully as i meant for the zobel resistors.
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Old 9th June 2009, 06:23 AM   #815
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My bad,
You will probably get away with 2W but I recommend using 5W.
Cheers,
Ian
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Old 10th August 2009, 02:42 AM   #816
netbug is offline netbug  United States
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Thank you very much Gingertube for this schematic.. it really is a wonderful sounding amp... now on to the cry for help

I finished my baby huey build and have run into an issue....

My B+ is 340. I read that Gingertube has said that 340 is the max, but what is the down side? It really sounds wonderful so I don't think I'm having any issues. Then again I'm a worrier.

am I going to kill tubes in a month?

Basically is it worth adding a power resister to get down to 330, or should I just live with it?
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Old 10th August 2009, 07:34 AM   #817
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Just live with it.

You are still within the tube ratings.

Did you build the original version with the CCSs in the EL84 Cathodes?
If so you can change the current sense resistors (4 off) in the CCS'es from 16 Ohms up to 18 Ohms. That will reduce the output tube idle current and hence its power dissipation slightly and give you longer tube life.
The 18 Ohms is a bit better value when you have 340V B+.

If a fixed bias version then just set bias to get 35mA idle per tube.

Cheers,
Ian
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Old 10th August 2009, 09:16 AM   #818
dupyv is offline dupyv  Luxembourg
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Default UL-feedback instability

Gingertube,

I'm building my 2nd BabyHuey using James JS-6228H transformers.

First, I tested the fixed bias variant with feedback from the EL84-anodes.
The scope-trace on a square-wave showed no instability and a quick listening test confirmed that everything sounded ok.

Then, I converted to feedback from the UL-tap and modified the resistors to the 10k-9k1-10k scheme.
Here I'm having excessive ringing on square-waves I can't get rid of.
The shown scope trace is at medium power level, turning up the wick worsens the oscillation. The probe is attached to an 8R resistor
connected to the OPT's 8 ohm tap. The oscillation is independant
of the square-wave frequency.

Things I tried and checked:
- Grid-stoppers are right at the EL84 socket, a variation in value up to 1k had no effect on the oscillation.
- 100uF+220nF are soldered right at the OPT B+ tap, this addition had no effect.
- Swapped the EL84 tubes, this has no effect
- Increased/Decreased the value of the 9k1 feedback resistor, this had no effect.
I also tried your later 6k8-22k-6k8 settings to no avail.
- Suppression of the 10R common cathode resistor of the EL84. This has had the most notable effect so far:
- The suppression of the 10R provoked an oscillation of the upper *and* lower part of the square-wave
- The addition of the 10R made the oscillation on the *upper* part disappear
- An increase to 22R had no effect.


Gingertube, did you encounter similar issues during testing you UL-feedback variant ? Any more checks/tweaks come to mind?


I remember having had a similar issue a while back when testing the cheap Hammond 125E OPT in my 1st BabyHuey.
When the 125E was replaced by a better Hammond 1650E everything was ok then.

A test I've still on my list is to swap out the James for a known good Reinhöfer-OPT of german manufacture.
If the problem still persists, my wiring is perhaps at fault. If not, the James JS-6228 though having a good reputation, doesn't like
operating in this topology with UL-feedback.

Kind regards,

Yves
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Old 10th August 2009, 11:19 AM   #819
Yvesm is offline Yvesm  France
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Hi Yves,

Forget UL

Yves.
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Old 11th August 2009, 03:51 AM   #820
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Yves,

Traditionally zobel networks were used with Ultralinear, wired on each side between anode and screen tap. Additionally g2 stoppers in the screen grid connection were often required and counter intuitively it seemed that the better the output transformer the more the g2 stoppers (higher value resistor) were required. Insufficient g2 stoppers usually shows up as "squegging", burst of RF just after the turn overs on +ve and/or -ve peaks, so that does not sound like the problem you have observed.

In the unit I am running at the moment I found that taking the feedback from the UL taps meant that the speaker impedance curve was more critical than when feedback was taken from the anodes. I fitted 10R + 1uF zobels across the 4 Ohm speaker taps. This helps linearise the speaker impedance with frequency BUT also its reflected impedance did at least part of the job the zobels from anode to screen would normally do. You could try this. You can also try increasing the g2 resistors.
You could also try zobels from anode to screen tap, I'd try a resistor value of 1K and capacitor of 1nF as a start point and then just change the cap value trying 470pF or 2n2 as required.

The other thing I've done in the unit I'm running right now is an interim thing. I added a Maida reg from B+ and then added 390K from the Maida Reg Output to each anode of the diff amp and doubled the diff amp current. That was just to get the diff amp triodes running in a more linear part of their Ia curve so that it did'nt "grunge up" as much when pushed hard. Eventually those 390K will be replaced with current sources or I may try to see if a can implement current mirror loads on the diff amp.

I have been a bit quiet while I built a new electronics workshop. That is now complete and I should get back to more work shortly.

Cheers,
Ian
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