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Old 15th November 2013, 02:58 AM   #1
tcqanh is offline tcqanh  Viet Nam
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Default tube 4-125A design

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.

Last edited by tcqanh; 15th November 2013 at 03:07 AM.
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Old 15th November 2013, 07:31 AM   #2
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Check out this thread. And Ale's great blog on his 4-65A SE amp.

Last edited by jazbo8; 15th November 2013 at 07:39 AM.
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Old 15th November 2013, 08:21 AM   #3
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by jazbo8 View Post
Check out this thread. And Ale's great blog on his 4-65A SE amp.
TkS Jazbo8,

I plan to use 4P1L for driver. AC heater for both driver and power tube. CT filament of 4P1L connects to ground, Filament Pot for 4-125A is adjusted on noise canceling.

HT maybe regulated by 6D22S.
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Old 15th November 2013, 05:14 PM   #4
mogliaa is offline mogliaa  United Kingdom
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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
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Old 16th November 2013, 12:21 AM   #5
tcqanh is offline tcqanh  Viet Nam
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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

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

Last edited by tcqanh; 16th November 2013 at 12:29 AM.
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Old 16th November 2013, 01:24 AM   #6
tcqanh is offline tcqanh  Viet Nam
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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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File Type: jpg 4-400sch2 G2 positive.jpg (56.4 KB, 232 views)
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Old 16th November 2013, 09:06 AM   #7
mogliaa is offline mogliaa  United Kingdom
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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
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Old 16th November 2013, 10:22 AM   #8
hpeter is offline hpeter  Europe
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Quote:
Originally Posted by tcqanh View Post
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
*******************************
*******************************
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Old 17th November 2013, 01:22 PM   #9
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by mogliaa View Post
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?
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Old 17th November 2013, 06:54 PM   #10
mogliaa is offline mogliaa  United Kingdom
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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...
Class A2 Direct MOSFET Coupled SE
cheers,
Ale
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