Recovered OT (for headphone amp ?) a question

[kagliostro]

I recovered (from an old military apparatus) a pair of small chassis (sort of audio Amps)

the tubes are 5654 and 6AQ5

The OT is labeled

TF - 1A - YY
Z 185+600:6000
(1-T-2):(3-4)
25 MA DC UNBAL
01,-20KC + 25DBM

so may be usable for a small headphone tube amp (using a 600ohm headphone)

I was just looking to the 6AQ5 spec and noticed that with 250v B+ and 5k load the current is the order of 45mA



Too much for the OT that is rated 25 MA DC UNBAL

I've no way to know which was the B+ voltage applied on the apparatus, there the OT impedance was 6k


but how can I discover which will be a correct B+ voltage level ?

Thanks

Franco

[PRR]

Rough approximation for pentodes: V/I should be similar to load impedance.

25mA = 0.025A

0.025A * 6,000 Ohms is 150 Volts.

Check.

0.025A*150V is 3.75 Watts Pdiss in the tube.

Audio output is perhaps 0.4 times Pdiss for high THD. 0.4*3.75W= 1.5 watts out.

Transformer rated +25dBm.  This is 0.316 Watts "max" output from the OT, for that frequency range and THD (not specified here).

The 150V 25mA condition appears to be 5 times more than the OT's rated audio performance. Or roughly twice voltage.

IMHO it is very likely that this is a "high quality" OT, that a 6AQ5 worked to the MAX is not "high quality" (5%-10% THD), and that the design works fine at 0.3 Watts out with good low THD, with about twice voltage available for peaks which are not low THD but not rudely clipped-off.

6AQ5 is about 3 times bigger tube than needed, but was VERY common and will be very reliable at 1/3rd of its max rating.

Several of the lower-Mu small triodes would also do this job.

0.1 Watts is LOUD in most headphones. You won't even be pushing the +25dBm spec. Bass THD should be good. You could even reduce B+ to 100V, even 50V, and still make a good sound.

[kagliostro]

Thanks PRR

A doubt, I didn't understand if your council was for a 150v B+ or you intend a lower voltage

Using the original tubes (5654 - 6AQ5) which is your council for B+ ?

---

Reading what you say I think it will be interesting to reverse engeener the schematic and to post it here for councils, originally I was thinking to reuse only the OT, sockets and tubes ...

Grazie

Franco

[PRR]

I don't see any reason to go over about 150V. As you note, this suggests too-large current for the OT.

Under 100V, design gets trickier.

So what is convenient in that range? To my eye, a 120VAC winding rectified to 160+VDC and filtered to 150V-120V DC.

[trobbins]

Franco, you may want to do some simple tests to get a better awareness of the OT. 

A primary inductance test with no DC, but varying the applied VAC to see how the inductance changes (it will increase as core perm increases, and then decrease as saturation on the peaks starts to kick in).

Also I wasn't sure about the secondary winding setup - you may want to confirm the turns ratios are equivalent to 185 ohm and 600 ohm for each half-winding (ie. total secondary is 1450 ohm).

Ciao, Tim

[kagliostro]

Many Thanks PRR and Tim

OK for B+ voltage 

I'm in difficulties about what concerns inductance test and relations between inductance of the OT windings and performances (too little brain for so much things related with an OT efficiency)

Better about to verify the exact situation about the turn ratio about which I can investigate easily

on the OT it is reported 185 ohm + 600 ohm and I was thinking to use the 600 ohm portion only because my knowledge about HiFi circuit is really poor, but I know there are 600 ohm headphones

I must verify the circuit to see if the 185 ohm winding is used for an NFB, one more reason to reverse engeener this small schassis

Thanks again

Franco

[Paul1453]

If you have 2 of these OTs it might be the beginnings of a very nice stereo headphone amp.

I simply love those little 6AQ5/6005W tubes!
They sound great to me, and they are available cheaper than other output tubes.

If you paired those with a couple of high quality 6DJ8s I think you would be quite pleased with the sound.   

[kagliostro]

I've a pair of chassis with those OT and also I'm sure I've some ECC88 and 6N23P

but currently if I find the time to do it, I wan to reverse engeener the circuit on the chassis that has a 5654 + 6AQ5

Franco

[kagliostro]

The chassis are labeled GS55112 and seems to be in relation with this thing

http://www.asap-aviationprocurement.com/rfq/american-telephone-and-telegraph/5996005033357/gs55112/

I didn't find a schematic, so if I want it I must go on with the reverse engeenering

and as it is a very compact montage I think that it will not be easy









Franco

[Paul1453]

I don't see 2 OTs on this.  Do you have 2 of these units?

I couldn't tell from the pic but am guessing the biggest box might be the OT?   

[kagliostro]

I've two identical chassis

yes, the big box is the OT, the cilinders are capacitors, the other box are filters or other transformers (no PT on the chassis)

EDIT: I've just find this description of the item

https://nationalstocknumber.info/part-number/GS55112_005033357

GS55112 Audio Frequency Amplifier

Technical Characteristics

Average Power Rating Per Channel (BHTJ): 1.0 milliwatts output
Basic Shape Style (AGTA):Rectangular or square
External Dc Operating Voltage In Volts (BKFB): 200.0 nominal single voltage
External Ac Operating Voltage In Volts (BKFC): 6.3 nominal single voltage
Frequency Per Channel (BHTB): 20.0 kilohertz maximum operating channel
Frequency Per Channel (BHTB): 3.0 kilohertz minimum operating channel
FSC Application Data (ZZZV): Telegraph power supply am682tcc3
Circuit Construction Type (BHSX): Electron tube
Operating Ac Frequency In Hertz (BJBQ): 60.0 nominal single voltage
Overall Height (ABKW):4.500 inches nominal
Impedance Rating Per Channel (BHTN): 0.600 kilohms output channel
Overall Length (ABHP): 5.125 inches nominal
Overall Width (ABMK): 4.750 inches nominal
Phase (FAAZ): Single
Variation Frequency Rating In Decibels (BHTG): 1.0

Read this, there isn't interest on reverse engeener the circuit, I'll use only the components for a new build

Franco