120w mains transformer with 4 x EL34's

[phillyhudson]

hi all
I have a set of partridge trannies pulled from an old 70's looking spare/repairs find that both work fine , the amp they came from had 6 EL34's so I'm assuming it was 120W - I remember I had to add a voltage doubler to the bias tap when I was using them in 4 x el34 amp , but I just wanted to check with someone who's knowledge is greater than mine [most people on the forum !] that I can build a new 4 x el34 amp , or a 4 x 6L6 amp with these trannies without any other concerns ?

I'd also appreciate an appropriate choke recommendation

the circuit will probs be a 183 or HRM version

thanks,

phil

IMG_3893.jpeg

[wpaulvogel]

I think those come out of a Sound City Mark 4 120. If this is what they are, the schematic shows 350 volts AC from the power transformer, great for your application. The schematic shows 375 volts DC on the B+ at the plates of the EL34’s. Wondering what the impedance is for the output transformer. These should cook!

[Stevem]

Sounds like you have a transformer set from a late 60s to early 70sSoundcity 120 watt monster amp.

In the band I was part of in 74, both the lead and 2nd guitar players had these amps.

They where just being imported into the US then if I am not mistaken.
If I recall right the V+ the power trans will supply will be too high to run 6L6 type tubes.
Also there far lower needs for filament voltage will kick your 6.3 bolts up too high and just eat up tubes on you.

Even though the OT is looking for 6 EL34s it will work good with just 4.
But again, to me the filament voltage issue should be addressed.

To that end case just wire up two more sockets for heater only, for the sole purpose of having any kind of octal pin output tube in there to soak up some of that excess filament voltage and your golden!

[phillyhudson]

thanks for advice - is there any alternative way of reducing filament voltage ? - the chassis they're going in doesnt have room for more sockets

[martin manning]

I'd start by measuring the unloaded voltages on the PT, and finding out what the OT primary impedance is.

[Phil_S]

thanks for advice - is there any alternative way of reducing filament voltage ? - the chassis they're going in doesnt have room for more sockets

First, answer Martin's question. Then we can work on reducing filament voltage if needed. Voltage can be reduced with a resistor in series or with diodes, but both of these methods generate quite a bit of heat. After all, you are converting voltage into heat. The energy has to go somewhere.

[wpaulvogel]

I’ll almost bet the filament voltage will be acceptable. Measure it unloaded. It’s already a high current winding and because it’s going to be a really low DCR it’s probably about 6.6 volts unloaded and the draw from 4 EL34’s is going to pull it down.

[phillyhudson]

Sorry for delay - I didnt have a low voltage AC source so ended up using a 5V tap from another amp , so here are my measurements for unloaded trannies….
Bias 33v
HT 354v
Heaters 6.7v

OT - 5.1V applied and 51v/102v/152v measured between secondaries , so that would make the winding ratio 100/400/888 respectively , and the primary impedance 3.2k - I think that’s right but my maths is terrible…

[martin manning]

If I understand correctly
152V means (152/5.1)^2 * 4 = 3k5
102V means (102/5.1)^2 * 8 = 3k2 and
51V means (51/5.1)^2 * 16 = 1k6 (and this looks odd)

354VAC with a FWB will get close to 500V B+, which is not great with 4x EL34 and ~3k4 primary. Does the OT have screen taps? If not, maybe consider a screen voltage reducer.

The heater voltage looks like it will be fine with either 4x EL34 (6A) or 4x 6L6 (3.6A).

[Phil_S]

OT - 5.1V applied and 51v/102v/152v measured between secondaries , so that would make the winding ratio 100/400/888 respectively , and the primary impedance 3.2k - I think that’s right but my maths is terrible…

The turns ratios are 10:1, 20:1, and 29.8:1. The impedance ratios (square of turns ratios) are 100, 400, and 888. At this point you can assume a primary impedance of 3.2K and that says you have speaker taps for 32Ω, 8Ω, and 3.2Ω. I am thinking, perhaps, it is better to assume primary impedance of 1.6K, which gives you speaker taps for 16Ω, 4Ω, and 1.8Ω. (You can assume 1.8Ω is 2Ω and 3.2Ω is 4Ω.) What you assume for primary impedance really depends on what you want for speaker taps.

Meanwhile, your filament voltage is right where you want it to be, so no worries about dropping any excess voltage.

[Phil_S]

Martin: I see we each took a different approach here. FWIW, I also came to B+ around 500V. If this set was running 6x EL34, I'm thinking this information suggests a primary impedance somewhere between 1.3KΩ and 1.6KΩ. When I run the math, 1.6KΩ primary seems to fit perfectly for all 3 of the secondary taps. Although 2Ω cabinets are not typically, they are not unheard of. Does this make sense to you?

[martin manning]

I'm looking at this: https://irationaudio.com/2015/03/26/sou ... 0-mark-iv/
Looks like the taps are 4, 8, 16, and 100V (which would be 32Ω to maintain Zpri). The plate and screen voltages seem low... I see that a DC voltage is shown. What RMS voltage does the HT on the PT actually measure?

[phillyhudson]

thanks for answers - so if I understand correctly it'll be ok if I use a screen voltage reducer ?
also , what's easiest method for identifying ground on OT secondary outputs ?
no colour-coding here

IMG_3894.jpeg

[martin manning]

You have just four secondary leads, then?

1. Find the pair that has the largest resistance. One of those is the common, and the other is the highest impedance tap.
2. Put some AC on the primary.
3. Choose one of the two leads identified in step 1 as common, and measure the voltages to the other three.
The other one of the two identified in step 1 should have the highest voltage, confirming that you have identified the two ends of the secondary.
4. Choose the other of the leads identified in step 1 as common, and measure voltages to the other three.
5. Arrange the two sets of measurements in ascending order (common is 0V) and look at the ratios between voltages.

From there you can tell which is which.

Can you confirm the PT's HT AC voltage?

[phillyhudson]

that's really helpful thanks , I've just re-measured HT and 5.1v in and 7.62v out so mains supply is 230v divided by 5.1 = 45.09 , then multiply that number by 7.62 = 343.5v on HT - I think that's right, so COMPLETELY different from my original measurement !