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1.5K 200W output transformers? What tubes?

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Miles Prower

Miles Prower

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Joined 2005
dolphin-hsl.com
battradio said:
The trouble with most sweep tubes is the low screen voltages , they need not to good for ultra linear operation , buy a bunch of cheep 6L6GC's and sort them .
Click to expand...

I'd go with the sweep VTs for audio finals. I did one project with 6BQ6GAs, relative pipsqueaks as compared to some of these beasts made for color TVs, and still get nearly 40W. At least I would, but for using an OPT that's rated for 30W, and seems originally intended for Class A, PP 6L6s, but the 4K4 (P-2-P) primary works very well with the 6BQ6s. No audio use mentioned in the spec sheets, but they sure do sound quite good even before applying the gNFB.

I have a preliminary design for a really big amp (attached). I have one NIB 36LW6 in my collection already, and some more on the way. Nice thing about this is that Edcor already has an OPT that'll work with these as a stock item. The peculiar heater voltage is somewhat problematic, but I figure I'll wind my own PTX for that: heater volts and a secondary for the negative rail voltages.

Another nice thing about HD VTs is that UL isn't necessary. These are linear enough so that all you need is some gNFB to take the edginess off and improve woofer damping. 6BQ6GAs sound as good as the famous 6V6, yet can out out almost three times the power.

Also, vertical deflection pents for color TV aren't bad either as audio finals. Some even include a small signal triode in the same bottle: integrated cathode follower grid drivers, or LTP use.
 

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E

es345

Member
Joined 2012
deicide67 said:
I'm in the process if acquiring a pair of 1.5K 200W outputs. They have 40% taps.

...

I have a few options for PS voltages.

A 1.2KV CT transformer that can be used with choke input for 520-560V at 500mA. Obviously KT88 or 6550 variety for UL here.

A pair of transformers that will probably do 3A or so at around 330-360V. Thinking 6L6 or EL34 for this.

Suggestions?

Thanks!

Blair
Click to expand...

some comments

1. regarding the power supply:
An efficient PPP design will have an efficiency factor at full power of about 66% (Anode power dissipation only, without G2 dissipation) So if you want to have 200W output you need more than 300W supplied power

a) PT1 520-560V 500mA: my understanding is that the voltage is DC. Then you have a supplied power of 260-280W. That might be at the lower limit, just enough for audio applications

b) PT2 330-360V 3A: my understanding is that this is the AC voltage. No problem with the supplied power possible. The rectified volatge under full load is about 1,2 of the AC voltage >> ~400 -430V

2. Output power

For some variations you might use my estimation excel for Push pull output stages stored here Smartest way to 200W? - Page 2.

PT1 voltage will allow more than 200W, on the other side there is the limitation of the current supplied.

PT2 voltage requires very low saturation voltage to reach 200W. So using sweep tubes might be the way to go.

3. Ultralinear:

If the max voltage of G2 is much below the targetted plate voltage you can still do UL , if you have a fully separated G2 winding at the OT. I have done this for a 200W power stage using KT120 at Ua of ~800V and G2 at ~600V. you need to scale the percentages along the voltage ratio. In this case e.g. a 40% UL is then 53%.

Hans- Georg
 
D

deicide67

Member
Joined 2008
Hi Georg,

I will have to look at your zip file on a computer. I'm on a dumb phone:)

PT two is the DC voltage. It can be used as a doubler for 680V or so. Originally it powered 8 X 8417s, so it's up to the task.

I just figured at 340-360VDC, it would be good for 6L6 varieties in UL.

Thank you,

Blair
 
G

gorgon53

Member
Joined 2006
Paid Member
If you can settle for 80W there is also the choice of 2 EL3010 tubes with 150V on g2 and 375V on the anodes. Ia=2x60/200mA, Ig2(0) = 2x6,5/36mA, Ug1= ca -9,3V.
thd is up to 60W below 3% rising to 5% at 80W. This all can be driven with only 5,5Veff
to each grid. You will need stabilized g1 and g2 voltages, g2 needs also overcurrent protection to prevent meltdown in case of severe overdrive.
 
G

gorgon53

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Joined 2006
Paid Member
Offcourse with four EL3010 you would get ca200W but you need 500V on the anodes.
But I would not recommend paralelling those tubes, beeing that steep they can be tricky.
If you need a soundpressure unobtainable with 2 tubes I would recommend paralelling speakers instead.
 
G

gorgon53

Member
Joined 2006
Paid Member
Most propable the transformer was originally designed for GEC KT88 tubes Class AB1, Cathode Bias, 40% tapping.
The data for a pair of tubes is as follows:
Va,g2(0) 500V
Va,g2(max sig) 436V
Ia+g2(0) 2x87mA
Ia+g2(max sig) 2x99mA
RL(a-a) 6kohm
Rk 2x600ohm
-Vg(aprox) 52V
Pout 50W
Dtot 1,5%
I.M. 4%
Pa+g2(0) 2x38W
Pa+g2(max sig) 2x17W
V(g1-g1)(ac) crest 104V
Zout 4.8kohm
It is essential to use seperate cathode resistors
Intermodulation distortion measured using 2 input signals at 50 and 6000Hz (ratio 4:1)

looks like 4 pairs KT88 would make a nice amp with but unless the transformer weigths in at at least 12kg+ I would think it is dimensioned for PA use
 
D

deicide67

Member
Joined 2008
Everyone keeps pointing to the EL509. It looks like a nice tube, and I can feed it 3-350V for UL, or 540V for pentode based on my transformers.

With a 35W dissipation, why would it be better than the 40-42W dissipation of the 6550 or KT88?

Will a quad of them perform better into 1.5K than a quad of KT88s?
 
D

deicide67

Member
Joined 2008
Yes, the EL509.

If I do a FWB, I should have around 330-350v for the plates. Plenty of heater current.

Is 1.5K A-A too low for a quad of EL509s? What would be the optimal bias current if I used fixed bias?

Do the top plate 509s do as well as the JJs?

Thanks for your help!

Blair
 
M

mcs

Member
Joined 2003
stiftsbogtrykkeriet.dk/~mcs/
deicide67 said:
If I do a FWB, I should have around 330-350v for the plates. Plenty of heater current.
Click to expand...
The official screen grid rating is 300V, but 330-350V may work also - you have to try for yourself ;)

Is 1.5K A-A too low for a quad of EL509s?
Click to expand...
It should be OK - but don't hook-up your 6 ohm speakers to the 8 ohm tap.

What would be the optimal bias current if I used fixed bias?
Click to expand...
At 300V, 100mA per tube is a good value. With the higher voltage, you should probably go down a bit. 85-90mA perhaps.

Do the top plate 509s do as well as the JJs?
Click to expand...
The original EL509s are only rated 30+7W. If you buy NOS tubes, look for EL/PL519 or the Svetlana 6P45S. They are rated 35+7W, like the new production JJ tubes.

Best regards,

Mikkel C. Simonsen
 
Fuling

Fuling

Member
Joined 2001
I have a pair of mono amps with 2x PL519 (similar to EL509) per channel. With Hammond 1650T (1,9k) output transformers and 360V B+ I get around 80W, though it seems that the p-p impedance is a bit on the high side as 4 ohm speakers sounds better connected to the 8 ohms tap. Higher plate voltage would result in (much) more power but I´m stuck with a specific pair of power transformers. The ciruit is similar to Pete Milletts Engineers amp, full pentode connection with Schade feedback.

A quartet of EL509 with 500+ V on the plates should be a good match for your OPTs, a single pair should also work well if you keep the voltage down.
 
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