How reuse this Mil Spec PT ?

[kagliostro]

I've just recovered a Decibel Meter ME-22A/PCM (unfortunately without the tubes and cover)

I think I can reuse the rack mount chassis, choke (8H 50mA) and some minor parts

The PT has a nice potted look and is interesting

but High Voltage with not many current put me in doubt if it can be reused for a small amp

The spec of PT are

primary
115v + 115v

Secondary
6.3v @ 1.8A
6.3v @ 0.9A
5v    @ 2A
750v CT @ 50mA DC  (375v-0-375v)

375v * 1.4 = 525v DC, not a low voltage as B+ and there is "only" 50mA DC current disposable

Any suggestion about a Final Tube and OT Impedance ?

Thanks

K

[sluckey]

I'd use it in a low current application, something like a preamp, revibe, warbler, etc. Reducing the B+ would be easy in a low current application.

[printer2]

Yeah, pretty useless. Where you located, if not too far Ill pick it up.   

And once I got it I would run 5Y3 and the first stage as a LC filter rather than a capacitor on the first stage. A capacitor fight after the rectifier gives you a greater voltage than the AC voltage out of the transformer, with an inductor you get a reduction as far as I can remember. Threw the transformer and inductor values into PSU Designer along with a couple of stages for screen and preamp tubes, a power tube load of 30mA and I get about 280V to 40mA with 260V on the output tube(s). So you can run with an OT in the 10k ohm range.

Should do a SE 6AQ6, 6K6 amp. A 12BH7, in p-p. Other tubes out there that fit that voltage and current requirement.

[kagliostro]

Thanks Steve & Printer2

@ Steve

That can be an option and as you say a voltage drop in a low consumption circuit can be achieved easily

@ Printer2

I agree that the use of the choke of the instrument (8H 50mA) with a 5y3 tube for a choke input rectification will result in a lower B+ instead of a capacitor input rectification

---

Thinking about the tubes I've that can afford high plate voltages I remembered I've a QQE04/20 and four 1625

I give a look to the 807 datasheet (same tube as 1625 the difference is in socket and heater voltage)

and I've seen the specs for 807

look to the first row in 807 Aplication Data, here

http://tdsl.duncanamps.com/show.php?des=807


in class A SE

Va       500
Vg2      200   
Vg1   -14.5
Ia       50.0
Ig2       1.6
Ra   39,000
S         5.7
Rk       280
Zout 6,000
Pout   11.5
THD      12

and so seems usable (or I'm wrong?), but THD at 12 seems to be too high 

K



 

[printer2]

Well the 807 is running at 500V with 50mA. You have at a 100% conversion, 375V with 50mA. Now if you were using a capacitor as the first element the voltage will go up, but there is no free lunch, the current will go down. So if voltage goes up 375V x 1.3 = 487V, then current would go down 0.050mA / 1.3 = 0.038mA. You are going to be a little off if you are going for center bias. With the preamp tubes and screens needing their own shair of the mA's the plate may only be getting 30mA.

The 12% distortion signifies the amp is in clipping, heck we probably operate them at 50% and like the sound.

[kagliostro]

The Pout reported is 11.5W, if I can obtain also only 5 or 6W I'll be very happy with that

---

About design I've think to a simple two tube amp like this

https://taweber.powweb.com/store/lauren_schem.jpg

or at least 2 preamp tubes plus the final tube

I think that a preamp with one tube will fit the transformer spec, and may be also two preamp tubes

---

What will happen with SS rectify ?

K

[HotBluePlates]

I've just recovered a Decibel Meter ME-22A/PCM (unfortunately without the tubes and cover)

I think I can reuse ... but High Voltage with not many current put me in doubt if it can be reused for a small amp

Look again at the schematic you posted.

The circuit is an input transformer, range switch, 3x 6SJ7 amplifier stages and a rectifier to convert the signal to d.c. for the meter movement. What you see at the bottom is a tube rectifier, simple power supply for the triode-wired 3rd 6SJ7, and a regulated supply voltage feeding the first two 6SJ7 stages.

The 6L6, 0C3 and 6SQ7 are your regulated supply. All that just to feed a pair of small-signal pentodes. The initial B+ is high (though reduced somewhat by the 5Y3) because it was always going to be reduced by the regulated supply. However, the 50mA rating is probably misleading as the original circuit probably didn't draw near that much.

I'd use it in a low current application, something like a preamp, revibe, warbler, etc. Reducing the B+ would be easy in a low current application.

I'm with Sluckey... except I'd only remove the 6SJ7's and the 6H6, replacing them with some form of preamp-type circuit. You already have a solid regulated power supply built which perfectly suits your PT and reduces the output voltage of the power supply.

[printer2]

Yeah but he has none of the tubes. So really he has some iron and some sockets.

With the Weber circuit and SS diodes I got about 400V for the plate with a 40mA bias. I would go for it.

[HotBluePlates]

Yeah but he has none of the tubes. So really he has some iron and some sockets.

He's gotta buy tubes either way, right?

Milspec 0C3 is $6.45, the 6SQ7 is $5-6, the 6L6 can be any variety (even the cheapest Russian variant). Nobody uses these tubes (except the 6L6), so they're really cheaper than the quoted retail prices on ebay or flea markets.

If it weren't for the fact the tubes would sit in Italian customs for 3-5 months, I'd just send Franco the tubes.

With the Weber circuit and SS diodes I got about 400V for the plate with a 40mA bias. I would go for it.

Most folks who repurpose electronics like this get very frustrated with the end result.

Generally you have to rate an input choke well in excess of the intended d.c. draw. That's because cap charging current has to flow through it, and are short and suprisingly-large pulses. If that input choke ever saturates, inductance falls to zero and the B+ shoots up to the value for a cap-input power supply. So you probably would have to buy a bigger & more capable choke to have a choke-input power supply.

If you're a betting man, play the odds. You have a much greater chance of a payday if you keep everything largely as-is with regard to power supply components and work with what you have.

Going for a complete gutting of the circuit to repurpose a couple bits of iron in a way they weren't used originally is like going all-in and needing runner-runner to make a straight; the cards are very much against a favorable result in a ground-up redesign to force parts into a circuit they weren't intended for.


I suppose the Weber circuit is doable, even with the too-high supply voltage. It's just that I foresee having to spend more money to get to an end result with that path than with the path I suggested. But if he really wants the SE 807 amp, then I guess that is the right choice to make.

[PRR]

Fender 6V6 Champ is 37mA at 250V to 400V. Some people play Champs.

BTW: this meter HAS a 250V *regulated* supply, at 6L6 cathode. 6L6 is easy; but if you have to buy the gas-tube and a 6SJ7 amp this gets costly; and regulation would probably make a Champ too "mild".

Yes, 520V on first cap, but that means stacked caps or oil-caps. Indeed you have 4uFd+8uFd of oil-cap, probably still good. But you want most of your caps to be <450v. So you want to drop voltage with a B+ resistor.

Drop a LOT, to like 300V-350V, for safe 6V6 Champ.

What's the "most" you can get out of it? Say 5mA for your little tubes, so 45mA for your big tube. At 450V, that's a 10K load, practical. Power input is 450V*0.45A= 20 Watts, perfectly practical in 6L6GC or EL34. Power output about 8 Watts. Not a teeny amp.

[printer2]

I remembered I've a QQE04/20 and four 1625

I give a look to the 807 datasheet (same tube as 1625 the difference is in socket and heater voltage)

Now if he can scrounge up a 12AX7 and some passive components...

If it don't turn out, heck it is a learning experience.

[kagliostro]

Many Thanks HotBluePlates - Printer2 - PRR

While you gave your answers and your suggestions, here was night and during the sleep time, a pair of possible (maybe) solutions I came to mind, I was wondering if a VVR with a trimmer instead of a potentiometer or a BIG zener can do the job lowering at the desired level B+

Now I didn't remember the values of the BIG zener I got the past year at a flea market, but I think among it I've one (or a pair to put together) that can do what is needed

but PRR put on my ear a flea

regulation would probably make a Champ too "mild"

so a VRR or a zener will tend to give the amp a not likeable tone ?

---

I must point that in the unit there weren't tubes, bu I think I've all those kind of tubes in my stock (may be except the 0D3 tube)
(the past year I give my support to a friend that had a stock of military instruments and equipments helping him at flea market to sell or exchange with other, so as present I received some now unusable military equipment with a lot of interesting parts and tubes on it that I recovered)

But honestly I must say that I consider to use this chassis (rack U4) for a preamp a waste of space, I'll prefer to built there a 15-30w amp in the future

To me seems (may be I'm wrong) that to rebuild the PS as is with the 0D3 and especially the 6L6 tube is a "waste" of tubes

---

About the oil caps I can read the 8uF and it is labeled 600WVDC, and may be the 4uF has the same voltage rating I think that those are a pair of good capacitors

---

So you want to drop voltage with a B+ resistor.

Drop a LOT, to like 300V-350V, for safe 6V6 Champ.

What's the "most" you can get out of it? Say 5mA for your little tubes, so 45mA for your big tube. At 450V, that's a 10K load, practical. Power input is 450V*0.45A= 20 Watts, perfectly practical in 6L6GC or EL34. Power output about 8 Watts. Not a teeny amp.

Here I need a bit of an explanation

450 / 0.045 = 10,000 Ohm

but didn't I must use the previous 300v-350v for the computation ?

350 / 0.045 = 7,777 Ohm

or you were referring to drop to 450v and use 6L6GC or EL34 tubes with 10K OT ?

Many thanks to all again

K

[HotBluePlates]

... I was wondering if a VVR ... can do the job lowering at the desired level B+

... to rebuild the PS as is with the 0D3 and especially the 6L6 tube is a "wast" of tubes ...

Understood.

It is worth noticing that the power scaling circuit or VVR is basically a solid-state version of the existing tube regulator circuitry, which could have a fixed or variable voltage dropped across the series pass element. In other words, the MOSFET of the VVR or power scale is doing the same function in the same place as the 6L6 in your meter.

So using a VVR or power scale would be the same thing that I was recommending, but save on tubes and space at the expense of getting the new components.

[tubenit]

How about a push/pull 6SN7 amp?  2 watts and it should get you under the 50ma current capabilities.  Use a 5Y3GT rectifier.

With respect, Tubenit

[kagliostro]

Thanks HotBluePlates & Jeff

@ HBP

You are true, but I can think to devote a mosfet to the voltage regulation, a tube, for a small amp, will be too much

@ Jeff

You want me to have a larger job than my initial idea 

but as your proposal is for a "reduced" version of a Mars ODS I think I can find the Layout in some your previous post here in the forum (isn't it?)

Have you considered we have more than 500v B+ ?

You are tempting me .............. just to know ................

....................... using a 6SN7 wich will be the OT primary impedance ?

Franco

[tubenit]

375-0-375

375 X 1.1 (5Y3GT using the 5v ) = 412.5.  Still high but maybe would work.  You could easily dial the LTPI and power tubes 6SN7 down to 275 volts with the VVR.

Just an idea to consider. Not pushing any agenda. 

I suggested it because of the 50ma current and thought this idea might come under that spec. 

You could easily do a HoSo56 with 6SN7 power tubes also.  Much less complicated and a fantastic build!  There is obviously a layout already existing for this one also.

With respect, Tubenit

[printer2]

So given the high voltage to begin with why not use the 1625 already on hand? I just looked up the Fender Champ 12 and they are running 495V at 44mA through the cathode resistor.

http://ampwares.com/schematics/champ_12.pdf

[kagliostro]

Thanks Printer2

I didn't remembered the Champ 12 with his 6L6 tube

interesting, may be they have those PT and used it with a doubler

if a 6L6 tube can afford the 480v on plate (496v on the first node)

I think I can build something also with that high B+ without the use of a VVR

May be an el34 will be less prone to have problems with this high voltage

so now I've a great confusion, I must think more about what to do

6SN7 - 6L6 - EL34 or 1625 ???

-------

Someone knows the transconductance of a 1625 tube ?

Thanks

K

[DummyLoad]

Someone knows the transconductance of a 1625 tube

it is an 807 with 12.6V heater.

--pete

[kagliostro]

Ciao Pete

Yes it is, but in the datasheet I've seen I wasn't able to find the Transconductance data

Thanks

K

EDIT: I've just downloaded a very complete datasheet for the 807 tube, if someone has interest in this tube, don't miss to download it http://www.retrovox.com.au/STC807.pdf

[PRR]

>> Drop a LOT, to like 300V-350V, for safe 6V6 Champ.
>> What's the "most" you can get out of it?
> Here I need a bit of an explanation
> but didn't I must use the previous 300v-350v for the computation ?

Sorry, I should have said OR.

You can drop to ~~350V and do a regular 6V6 Champ....

*OR*

You can take the full 450+ Volts and most of the 50mA (say 45mA) to do a "most you can get from this PT" amplifier.

450V*45mA is 20 Watts of heat. That's too much for 350V 14W 6V6, but is entirely acceptable in any recent "6L6", 807, or EL34.

OT should be near 10K SE, power output about 8 Watts.

A push-pull amp would need two 10W 450V tubes and a _20KCT_ 10 Watt output transformer. This is awkward. Something like a HI-FI 7189 "20 Watt" OT could be re-loaded with 8 ohms on the "16" tap and give good performance, but 7189 is 440V max and hard to get; EL84 is the same tube rated 350V and while recent EL84 will take more than 350V, 450V is mighty high. 6L6/EL34 will work but are awful big for a push-pull 10 Watt amplifier. So I don't think Push-pull is appropriate.

[kagliostro]

Thanks PRR

Now I can follow you with more clarity

Ideally, instead of a 6v6, I would prefer a tube that can afford an higher B+, one of those you mentioned
("6L6" - 807 - EL34)

---

On the 807 datasheet I've linked in my previous post there are indication for AF use in SE till 500v B+ for fixed bias and cathode bias (see below extract of the this datasheet http://www.retrovox.com.au/STC807.pdf)

Saying that I like more cathode bias architectures

What do you think to use the stated (see datasheet) 6k OT at 500v B+ with a cathode resistor higher in value than the 280R reported (see datasheet) as to tame the mA consumption within the PT specs ?

Or it will be better to use a fixed bias and use the bias regulation to tame the consumption ?

K