Working with a Hammond 270HX 550VCT.. Need to get 450V B+

[thehoj]

So I'm potentially going to be working with an amp that's using a 270HX PT.. I'll be using it in an amp that'll be running 4 x 12ax7's and 2 x el34's in a Push pull configuration.
The PT is 550VCT @ 230mA, and so using a full-wave rectifier (550*0.707 = 388V) I imagine this would work out to about 325 - 350V under load with 4 12ax7's and 2 el34's.. Ideally I want to be running 450V B+ under load.. Now I could do a full-wave bridge rectifier (550*1.414 = 777V) and that would be what maybe 700V under load?

Is there some way I could make this work for my needs?.. Would it be crazy to use some zener diodes on the CT to drop 250V??.. That does seem kind of crazy now that I think about it.

Any thoughts on this? Or is this PT just not going to work for my needs?

[thehoj]

NTE makes these reverse polarity zener diodes up to 200V 50W.. http://www.galco.com/techdoc/nte/nte5296ak_dat.pdf

As far as calculating the power dissipation that this zener would experience.. would I use the total current the amp would be drawing?.. So if say I was running at 500V, and at max my el34's are drawing 50mA each + say 20mA total for the 12ax7s, 120mA would be somewhat accurate for total amp current draw?

If it is then the zener would see .12 * 200 = 24W..

Has anyone ever dumped this much Voltage before in a similar setup?
Am I going about this in a realistic way?

[jazbo8]

The large voltage drop and high power dissipation in the zener(s) make this a poor choice imo, instead, try and design around it or get another PT suitable for the task, and save this PT for another day. Is the B+ of 450V a must in this design or are there some flexibility?

[thehoj]

The large voltage drop and high power dissipation in the zener(s) make this a poor choice imo, instead, try and design around it or get another PT suitable for the task, and save this PT for another day. Is the B+ of 450V a must in this design or are there some flexibility?

This is kind of what I've been thinking also.. Out of curiosity though, do you say it's a poor choice because there'd be all that extra heat production from the zener? Or is it a poor choice because it would actually affect the way the amp works?.

Basically what I've been wanting to build is a Marshall 2204-ish clone.. Something along the lines of this https://taweber.powweb.com/store/8cm100_schem.jpg (minus 2 power tubes).

Maybe I will build it with this PT at the lower 325-ish V B+ and see how it sounds.. if it's not what I'm after I guess I can always replace the PT later. This PT just fell into my lap, so I'd like to make use of it, but I don't want to build any unnecessary amps for myself.

With 550VCT @ 200mA and 2xel34 what is a realistic B+ to expect under load? is 325V probably likely?

Actually, the PT has 115 and 125V primary taps, so if I used the 115 taps (ratio of 550/115 = 4.78) and I'm actually seeing 120 out of the wall, I guess I'd potentially see a bit more Voltage (4.78 * 120 = 573V).. That would help a little bit.

Looking up el34 datasheet info on tdsl shows a number of configurations, http://www.duncanamps.com/tdsl/show.php?des=el34
But all of them in that lower 300V range show the screen voltage higher.. Is it not common to run an el34 at 325V on the anode and the screen?

[jazbo8]

do you say it's a poor choice because there'd be all that extra heat production from the zener? Or is it a poor choice because it would actually affect the way the amp works?.

It's more of the former, too much wasted power in the PS.

With 550VCT @ 200mA and 2xel34 what is a realistic B+ to expect under load? is 325V probably likely?

That depends on how you bias them and what load you use, which we still need to calculate...

But all of them in that lower 300V range show the screen voltage higher.. Is it not common to run an el34 at 325V on the anode and the screen?

I too question those figures shown in the table, it is definitely not a good idea to have the screen voltage higher than the plate at idle ;-)

So what's the target output power for the build and the speaker that you plan to use? They will determine the correct operating conditions for the output stage.

Jaz

[thehoj]

It's more of the former, too much wasted power in the PS.

That depends on how you bias them and what load you use, which we still need to calculate...

I too question those figures shown in the table, it is definitely not a good idea to have the screen voltage higher than the plate at idle ;-)

So what's the target output power for the build and the speaker that you plan to use? They will determine the correct operating conditions for the output stage.

Jaz

I'm not overly concerned specifically about what power output number I could achieve for the build, but I'd like to have as much headroom as possible from the power stage.. In the end I want to achieve a lot of the overdrive created in the amp from the pre-amp section. I guess in my head I'm looking for that somewhat tight overdriven sound that you hear in a 2203 or 2204.
I would do a fixed bias, and generally speaking I've always biased for 70% plate dissipation with that type of configuration.
My speaker load is 2x8ohm WGS (ET65 & Vet30) speakers wired in series for 16ohm. I don't have an OT yet, so I could get whatever would work the best for as much headroom as possible.

[imaradiostar]

If you want tight, wire them in parallel and use an output with 4 ohms. The two in series will interact- not a bad sound, but usually not as tight.

You may find it's easiest to buy a pair of transformers that play nice together. Surf on over to the triode electronics web store and buy the marshall 50 watt set. They are excellent.

If you REALLY want headroom...don't build a 50 watter. I have a hunch you won't want as much as you say you do.

Jamie

[jazbo8]

Took another look at the Mullard datasheet again, I think your PT can run just fine with full wave rectifier, so B+ ~380V, the OPT should be around 4K:16. The output power and distortion will get worse if you veer too far from it.

[thehoj]

If you want tight, wire them in parallel and use an output with 4 ohms. The two in series will interact- not a bad sound, but usually not as tight.

You may find it's easiest to buy a pair of transformers that play nice together. Surf on over to the triode electronics web store and buy the marshall 50 watt set. They are excellent.

If you REALLY want headroom...don't build a 50 watter. I have a hunch you won't want as much as you say you do.

Jamie

Yea, I'm not looking for uber tight.. I just mean I don't want a real sloppy sounding amp.. I did not realize that about parallel vs. series wired speakers. I'll have to play around with that.

Also, it's not that I'm after a ton of headroom, but I do want to be able to make semi-clean sounds at a decent gigging (small club - not mic'd) volume.

[Gingertube]

So I'm potentially going to be working with an amp that's using a 270HX PT.. I'll be using it in an amp that'll be running 4 x 12ax7's and 2 x el34's in a Push pull configuration.
The PT is 550VCT @ 230mA, and so using a full-wave rectifier (550*0.707 = 388V) I imagine this would work out to about 325 - 350V under load with 4 12ax7's and 2 el34's.. Ideally I want to be running 450V B+ under load.. Now I could do a full-wave bridge rectifier (550*1.414 = 777V) and that would be what maybe 700V under load?

Is there some way I could make this work for my needs?.. Would it be crazy to use some zener diodes on the CT to drop 250V??.. That does seem kind of crazy now that I think about it.

Any thoughts on this? Or is this PT just not going to work for my needs?

I built a 4 x 6V6 Amp using 370FX (550V CT @ 173mA) and ended up with a B+ of 345V.

The 230mA rating is dependent upon drawing current from just one side of the centre tapped winding at a time. If you use a bridge rectifier across the full winding the current rating is effectively halved to 115mA. You MUST then leave the centre tap NOT connected. Also the 115mA current rating you end up with wont' cut it for a pair of EL34 in push pull so you need to change your aims or go for a different transformer.

A pair of EL34 cathode biased at around 340-350V B+ is good for 30 to 35 Watts and in my opinion at least sounds MUCH better than running EL34 in fixed bais at B+ of 450V plus trying to screw 55 watts put of them. I'll take the lovely 30W in lieu of the agressive 55 watts anyday. OT with Raa of 3K4 top suit.

Cheers,
Ian

[thehoj]

I built a 4 x 6V6 Amp using 370FX (550V CT @ 173mA) and ended up with a B+ of 345V.

The 230mA rating is dependent upon drawing current from just one side of the centre tapped winding at a time. If you use a bridge rectifier across the full winding the current rating is effectively halved to 115mA. You MUST then leave the centre tap NOT connected. Also the 115mA current rating you end up with wont' cut it for a pair of EL34 in push pull so you need to change your aims or go for a different transformer.

A pair of EL34 cathode biased at around 340-350V B+ is good for 30 to 35 Watts and in my opinion at least sounds MUCH better than running EL34 in fixed bais at B+ of 450V plus trying to screw 55 watts put of them. I'll take the lovely 30W in lieu of the agressive 55 watts anyday. OT with Raa of 3K4 top suit.

Cheers,
Ian

That makes total sense, I hadn't specifically thought about the implications of this on current draw.

[Zozobra]

A pair of EL34 cathode biased at around 340-350V B+ is good for 30 to 35 Watts and in my opinion at least sounds MUCH better than running EL34 in fixed bais at B+ of 450V plus trying to screw 55 watts put of them. I'll take the lovely 30W in lieu of the agressive 55 watts anyday. OT with Raa of 3K4 top suit.

Cheers,
Ian

I've seen this power amp arrangement mentioned a lot recently around these parts. Out of interest what sort of screen voltage is typically being used? I'd like to draw up some load lines to get some ideas of how to approach this circuit and bias it without melting anything for a project a friend had approached me about.

EDIT: I guess it looks a lot like the badcat hotcat30

[Mick Bailey]

I drop high B+ with a Zener/Mosfet combo in the CT. Works really well - Can't fault it. Two resistors, a Zener and cheap power Mosfet and you're there. All wired onto the Mosfet pins and use the chassis as a heatsink.

[ReadyTeddy]

So I'm potentially going to be working with an amp that's using a 270HX PT.. I'll be using it in an amp that'll be running 4 x 12ax7's and 2 x el34's in a Push pull configuration.
The PT is 550VCT @ 230mA, and so using a full-wave rectifier (550*0.707 = 388V) I imagine this would work out to about 325 - 350V under load with 4 12ax7's and 2 el34's.. Ideally I want to be running 450V B+ under load.. Now I could do a full-wave bridge rectifier (550*1.414 = 777V) and that would be what maybe 700V under load?

Is there some way I could make this work for my needs?.. Would it be crazy to use some zener diodes on the CT to drop 250V??.. That does seem kind of crazy now that I think about it.

Any thoughts on this? Or is this PT just not going to work for my needs?

That 777-volt calculation depends on large current pulses through the rectifying diodes to charge the filter cap(s) right at the peaks of the rectified sinewaves. Putting in some resistance or a choke before the first filter cap would cut down on that voltage pretty quickly--when the power supply is under load. The nice thing about a choke is that you are using inductance instead of resistance, so you'd be dissipating much less power.

[Gingertube]

I've seen this power amp arrangement mentioned a lot recently around these parts. Out of interest what sort of screen voltage is typically being used? I'd like to draw up some load lines to get some ideas of how to approach this circuit and bias it without melting anything for a project a friend had approached me about.

EDIT: I guess it looks a lot like the badcat hotcat30

Yeh! It does.

Here is one I built for the local Guitar God, a Junkbox Trainwreck. He - gigs with it all the time

Cheers,
Ian