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tube 4-125A design

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Hi all,

I'm planning to build 4-125A SE amp. My PSU can get 500-600V for anode and one separate winding for G2. I have some solutions for these tubes:

1. Anode runs 100mA at 500-600V, Vg2 is about 350-400V. Tube works like a normal tetrode. OPT will be 5K- 8 ohms.

1.1. G1 is still negative voltage. Tube runs in A1 class.
1.2. G1 is positive voltage. Tube works in A2.

2. G1 to ground, G2 become control grid and runs at few decade volts directed coupling from driver stage. Va is still 500-600V. Tube works like triode.

3. G2 to anode or Ultralinear winding. B+ is 500V or below for safe.

What is the best ?

Regards,

QA.
 
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Hi tcqanh,
Great project ahead indeed! I have my 4-65a SE running at 560V/100mA. A fantastic amp with a powerful bass and nice SE sound. Solid performance but have to admit that was overwhelmed with size of it. Anyhow, I don't want to scare you at all.

I don't think AC filament would be a good idea. You can get this amp dead quiet with Rod Coleman DC filament regulators, a must in DHT amps in my view...

I'd run the 4-125a in triode mode and forget about tetrode or ultralinear if the curves are nice in triode. If 4-125a behaves like its brother the 4-65a then you will have a low DF SE amplifier...
Cheers,
Ale
 
Hi Ale,

Your blog is so interesting. Tks.

This amp will be built in order to mini contest with its brothers, eg. 75TH, 100TH, 4-250a... in my group in new year. My daily amps are still 2A3, 300B and 4P1L alternating each others.

4-125a has 6.5A in filament, so filament board become too hot and has large heatsink... like mini solid state amp. It's quite hard to get Rod board cause internet banking is not popular in my stay. My power tranny has 5V 6.5A is not convenient for DC heater.

I did try noise-canceling to amp with 2 DHT tubes in AC filament, result is very good and hum noise is minimum. Will try again :D

I've built RS1003 SE amp in triode and pentode and I found how to make penthode amp sounds good :D. It will be applied for this project. Waiting suitable OPT...
 
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Ale,

How do you think what is going on if 4-65A or 4-125A run like schema below ? I think it is really interesting experiment :)

I will replace 10M45S to choke load.
 

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Hi TQ,
Thanks for your comments.
Regarding DC filaments, Magz is writing a fantastic experience whilst building his "Midlife Crisis" project with 833C's which have 10A filaments. Magz is succesfully implementing DC filaments with Rod Coleman regulators. Please read his thread at this forum which is very interesting.

Your design seems very interesting. I haven't tried myself screen drive and not even on a transmitting valve. Check Tubelab's website and several threads around here. You will get a lot of useful stuff. Surely you will need to swing many volts to drive the 4-125A. In my design I swing about 180vpp to get the 10W out in class A2. If I were to have a higher HT supply, you could get more power or run it only in class A1. I wanted to experience the sound of class A2 when properly driven, and I'm very happy with the sound of it.
You could replace the follower valve with a FET. Regarding the driver, it all depends on the gain needed. A nice pentode driver would be good. I've implemented a crazy stage with a step up transformer and a 46 in filament bias. Why? Because I love the sound of a 46 (in triode) and am a big fan of removing cathode bypass capacitors where possible.
Cheers,
Ale
 
Ale,

How do you think what is going on if 4-65A or 4-125A run like schema below ? I think it is really interesting experiment :)

I will replace 10M45S to choke load.
i think i seen that photo is some philipines forum
what is your expected power from amplifier?
i am planning to build something similar, but it seems for good eficiency and higher power , voltages must be high on these radial beam beasts
hope the calculations are right..
*******************************
*******************************
QY3-125A VARIAN (1400 85mA)
2250 500 0,200 0,01 120W
1750 0,19
9k2 41W A1
*******************************
*******************************
QY3-125A VARIAN (1400 85mA)
2400 400 0,2 0,01 120W
2000 0,19
10k5 47,5W A2
*******************************
*******************************
4-250A EIMAC (1400 140mA)
2600 400 0,3 0,02 196W
2200 0,28
7k8 77W A2
*******************************
*******************************
4-250A EIMAC (2000 100ma)
3700 400 0,25 0,01 200W
13k5 96W A1
*******************************
*******************************
 
In my design I swing about 180vpp to get the 10W out in class A2.
Ale

This is real problem. Big swing, big dissipation but small power out. With 10w output, I can go with 300B XLS KR easily. I want to get much more power so I choose penthode operation.

I did try 6e5p-eb for driver, it's great. I think cathode follower for it is good in this case cause of class A2. But its cathode voltage is quite low, maybe I need much more headroom driver tube.

Probably, 4P1L cathode follower with choke loading is the choice. I did pair of chokes 100H 50mA for longtime ago.

How is your driver circuit?
 
Hi TQ,
Indeed, large drive requirement is the price paid for triode operation. I cannot comment on the pentode mode and sound performance as haven't tried it that way.
The circuit I used was originally designed by Michael Koster and is DC coupled with a gyrator in the driver. You will see that A2 current has an independent loop and therefore the regulation requirements for this power supply are pretty low. Stacked supplies add complexity but the gyrator in mu-follower mode provides a very low output impedance on the driver making it ideal for A2 operation. The 46 driver can swing 200Vpp with less than 0.2% THD in this mode. It sounds great but requires a hefty filament supply. A nice experiment but won't recommend it given the onerous power supply requirements...
http://www.diyaudio.com/forums/tubes-valves/144767-class-a2-direct-mosfet-coupled-se-16.html
cheers,
Ale
 
Hi TQ,
Indeed, large drive requirement is the price paid for triode operation. I cannot comment on the pentode mode and sound performance as haven't tried it that way.

Hi Ale,

I've built 811A SE many years ago. It's true triode but it sounds like penthode. Why ? I don't know clearly :D, maybe due high impedance, high mu... these specs are the same of penthode tube.

I did assemble many penthode SE too, sound is so bad... unable to listen.

How to treat it ?

I've found some:

1. Zobel circuit with RC parallel to B+ and anode.
2. Cathode feedback winding to power tube or local FB from secondary winding of OPT to cathode's power tube.

Pls try it once you can because it's worth :)
 
've built 811A SE many years ago. It's true triode but it sounds like penthode. Why ? I don't know clearly
j2xgwDMBDSsnVA69OtCogRQzID4UOTOFEwAHkGJSmhPzgRIAFU7lagXmBJGuXwEwEMPlz8AaBEagAjDrBQAkcUj5iCtQwK0FSzhciBsQADs=
, maybe due high impedance, high mu... these specs are the same of penthode tube.
What impedance did you use? Nobu K Shishido recomends 3.5k for SE at 430 volts 100Ma.
i have made several 811 PP using 2.2K A-A PP {copied from record cutting Amp} best sounding amps i have made:compared to Kt88's 2A3's EL34's. i do think they sound much better PP than SE
Phil
 
What impedance did you use? Nobu K Shishido recomends 3.5k for SE at 430 volts 100Ma.
i have made several 811 PP using 2.2K A-A PP {copied from record cutting Amp} best sounding amps i have made:compared to Kt88's 2A3's EL34's. i do think they sound much better PP than SE
Phil

I did use 5K.

Impedance of OPT depends on operating point and tube internal resistance. The most important here is how to keep tube resistance low as possible.

Could you share your schema ?
 
811A SE and PP

Circuits fir 811A SE and Push pull.
I made a Se 811A a long time ago, but without the Inter-stage transformer.
Used a 6BX7 as the direct coupled Cathode follower.

The 811A at 600 volts last a very long time; you need to bias them at 70Ma.
You must switch the HT on after the 6BL7/6BX7 has heated up. Friend of mine has been using the PP 811A for 17 years{ Russian 811A's} without ever changing any of the tubes and using inefficient speakers .
Much longer than you will get from KT88/6550 amps.
Can drive very inefficient speakers.
I used an sowter 2.2k A---Anode, but works well with the hammond 1.9k A-A.
Goes very well with Altec and JBL highly damped drivers even with very little feedback, you need about 12Db feedback with speakers with wimpy magnets especially SE amps of any type.
Phil
 

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