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Problem using Hammond 126C IST

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6

6A3sUMMER

Member
Joined 2016
Note about the schematic in Post # 160

The 100mA meter sees all the instantaneous signal current of the 300B.
The meter sees both the AC and DC current.

If you connect the meter in series with the IXYS current source, then the Bypass Cap will take all of the instantaneous signal current of the 300B.
The meter will only see DC current.
 
R

rchamber

Member
Joined 2007
goddlediddles said:
Thanks for the compliment. No problems so far with oscillation. I'm including the complete schematic, with some notes, as well as the PSU schematic for anyone that wants to build it. I have replaced the power transformer with the Hammond 379X. I get about 9½ watts with the increased HT of 475 V.

I think it's a really good design, after trying many similar versions. All thanks to you who introduced me to the IXCP10M90S, for which I am eternally grateful.
Click to expand...

Hi, Came across this thread after following Andy Evans ventures with the 126C interstage. Very interesting to see that you dumped the 126C, continued along for a while with the 6SN7, then dumped it for 6J5...

Being an old guy from my first tube build in the 1950s, I am hesitantly trying to get with the program of solid state peripherals in my amp builds...your story about discovering constant current sources from innocence, learning about them and actually implementing them in your design is inspirational ... and so simple. Maybe enough so that I might try your design. A couple of questions:

1. You placed the IXCP10M90S in the plate circuit of the 6SN7, then 6J5. And you placed the output stage CCS in the cathode circuit. Why not in the output stage plate circuit?

2. Do you have any thoughts about next steps to make it better?

3. What is the rest of your system...sources, loudspeakers...?

4. Any special turn on sequencing?

5. Any thought about filament regulators for the 300B...e.g. Rod Coleman's regulators?

All for now, Best,

Robert
 
andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
rchamber said:
Hi, Came across this thread after following Andy Evans ventures with the 126C interstage. Very interesting to see that you dumped the 126C, continued along for a while with the 6SN7, then dumped it for 6J5... Being an old guy from my first tube build in the 1950s, I am hesitantly trying to get with the program of solid state peripherals in my amp builds...your story about discovering constant current sources from innocence, learning about them and actually implementing them in your design is inspirational ... and so simple. Robert
Click to expand...

I've tried adding solid state to my tube amps with mixed results.
- I still prefer a good plate choke to an interstage or an active load. A plate choke has a smoothness and detail that i can't match. I use Russian teflon FT-2 coupling caps, and they are very detailed and neutral.
- I tried Cree diodes in the cathode for a while and in the end took them all out - they added a hardness I couldn't live with. So back to unbypassed resistors with a plate choke of 150H inductance or similar. I can use resistors up to around 220R.
- Rod Coleman filament regs are 100% positive. Couldn't live without them. Here solid state is absolutely the way to go.

So that's it. For the driver stage an unbypassed cathode resistor of up to around 220R and a plate choke of around 150H. Amorphous is nice, but not essential. You can hear differences in plate chokes, but they're all pretty good, even a Hammond 157G can work. It's used in the Hagerman Tuba headphone amp.

I know that there are many fans of active loads and Cree diodes or red LEDs in the cathode. I'm not one of them.
 
goddlediddles

goddlediddles

Member
Joined 2010
rchamber said:
Hi, Came across this thread after following Andy Evans ventures with the 126C interstage. Very interesting to see that you dumped the 126C, continued along for a while with the 6SN7, then dumped it for 6J5...

Being an old guy from my first tube build in the 1950s, I am hesitantly trying to get with the program of solid state peripherals in my amp builds...your story about discovering constant current sources from innocence, learning about them and actually implementing them in your design is inspirational ... and so simple. Maybe enough so that I might try your design. A couple of questions:

1. You placed the IXCP10M90S in the plate circuit of the 6SN7, then 6J5. And you placed the output stage CCS in the cathode circuit. Why not in the output stage plate circuit?

2. Do you have any thoughts about next steps to make it better?

3. What is the rest of your system...sources, loudspeakers...?

4. Any special turn on sequencing?

5. Any thought about filament regulators for the 300B...e.g. Rod Coleman's regulators?

All for now, Best,

Robert
Click to expand...
Hi Robert. 1) The IXCP10M90S takes the place of the cathode resistor in the output stage. It means that I can get a guaranteed 72.5 mA plate current through the 300B regardless of what type of unmatched tubes I use. Of course, that means that the bias provided will not be the same for both tubes, and therefore the L and R drive required for full power output will be unequal. I thought at the time that the IXCP10M90S would prevent runaway if the 300B cathode resistor was very high (like 470K). As it is, 330K is a bit too high, according to the data sheets. It hasn't run away yet when severely over-driven with 330K. It was just an experiment, really, but I decided to leave it that way. I never use the amp at full power anyway.

2) I tried so many designs, before I settled on my own one. It performs well, so I don't see the need for any improvements.

3) I have a lot of speakers. I build them myself. I mainly use the Logitech media Server in a Raspberry Pi, into a Hifi Berry DAC and DSP, in
combination with a Raspberry Pi also.
It feeds a tube preamp which I built from a kit.
4) No special turn-on sequence - I don't think it's necessary. The HT comes up slowly, thanks to the tube rectifier.

5) Rod Coleman's and other similar regulators would load down the driver output impedance too much, thereby destroying the advantage of the
very high dynamic impedance of the IXCP10M90S, which combined with the 300B grid resistor (330K) provides a very high linear voltage swing
for drive to the 300B. You never run short of voltage drive for the output stage.

Thanks for commenting. Godfrey.
 
Last edited:
R

rchamber

Member
Joined 2007
Hi Godfrey,

In the interest of furthering my understanding of your amplifier I have the following questions:

1. How is the 6J5 heated...ac or dc?

2. Could you please indicate the Operating Points (Ia, Va-k and Vg} for the 300B output stage in the two latest schematics of posts 157 and 160?

3. Why did you opt for the higher Va, and also Va-k, with the higher voltage power transformer in you latest build (Post 160)?

4. Did you consider other than 3.5K output transformer primary impedance?

Thanks and Best Regards,

Robert
 
R

rchamber

Member
Joined 2007
Originally Posted by Rod Coleman View Post
Blocking distortion (charging of the grid cap during peaks) is much more of a problem in these little SE amps..

Hi Rod,

I'm trying to envision what a *grid cap* might be and I've looked for schematics that might show one...to no avail.

Please try to help me know what you are talking about.

Thanks and Regards,

Robert
 
Rod Coleman

Rod Coleman

Member
Joined 2004
Yes, that's the right capacitor. The mechanism is the same as described for guitar amps - but the effect is usually less dramatic.

With low-power SE amps, the grid (coupling) capacitor acquires a little charge if the power-valve grid voltage approaches the cathode voltage. It discharges again through the grid leak capacitor.

With recordings of large dynamic range (symphonies, for example), the peaks can charge the cap, to varying degrees - but it may not be audibly obvious that this clip has happened.

The polarisation of the charge is such as to bias the amplifier's end-stage colder.

What is perceptible, if you have built a fine-sounding amplifier, is that the sound loses something of its richness, especially in the quiet passages. If you lower the volume and pause the music for a little while (to allow the cap to fully discharge), it comes back sounding much better.

If the problem affects your amplifier/listening, it can be fixed with a DC-coupled follower - preferably a (carefully selected) FET source-follower.
 
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