OT for ST35

[bankeny]

Hello All!

I've been tinkering with tubes for a few years now and have built a couple of guitar amps. I've wanted to do a hifi project and think I have found a donor chassis suitable for a Dynaco ST-35. It was a stereo amp with two EL84's per channel. The output transformers are rated for 12 watts. Will that be a problem?

I can't find any information about them on the internet. They are made by "Quality Transformer & Coil Co. Chicago, IL" and it's a model number 6-W-17. Under the model number it reads "12 watts"

It does have UL taps on the primary and 4, 8 and 16 ohm output taps on the secondary.

Any reason not to use these in an ST-35? Hopefully they test out okay with no shorts etc.

Thanks!

Brandon

[bankeny]

Here's a picture of the OT in question.



Thanks!

[20to20]

The clue to start your project with is the power rating. Since it is a PP tranny rated at 12W it probably wasn't intended to be used with 6BQ5 tubes. Maybe 6V6. So you will have a power and probably an output Z mismatch.

[bankeny]

Thanks 20to20!

I haven't received the chassis yet. It was given to us along with some other "junk" (or so they though!) and my brother-in-law and I are splitting stuff up. I should get my share early next week. It's a stereo amp with ef86 -> ecc83 -> PP EL84 per channel. I'm wondering if it isn't based on the Mullard 5-10 as it has the same tube component. Will know more next week!

[kheper]

My guess is that they will work fine, especially since they were used with el84s. To take the stress off of the power tubes and outputs, you may want to use the 4ohm tap with 8ohm speakers; Doing this will increase the input impedance. You could drop the B+ and idle bias to dial the audio output to ~12W. The ST35 is billed to do 17.5W, but my guess is that its output is ~15W. A lot of older PP el84 amps (like Heathkit) were spec-ed at 12W.

[Gingertube]

Brandon,
2 x 6BQ5 / EL84 in Push Pull Ultralinear will give 12 watts max., more typically about 10.5 watts with B+ in the 300 to 320V range. ( The 17 or 18W specification is for 2 x 6BQ5 / EL84 in Push Pull PENTODE Mode ONLY)

Those transformers will be fine without messing about with putting 8 Ohm speakers on 4 Ohm taps, dropping the voltage rails or anything else. , just use Ultralinear Mode..

Cheers,
Ian

[kheper]

Brandon,
2 x 6BQ5 / EL84 in Push Pull Ultralinear will give 12 watts max., more typically about 10.5 watts with B+ in the 300 to 320V range. ( The 17 or 18W specification is for 2 x 6BQ5 / EL84 in Push Pull PENTODE Mode ONLY)

Those transformers will be fine without messing about with putting 8 Ohm speakers on 4 Ohm taps, dropping the voltage rails or anything else. , just use Ultralinear Mode..

Cheers,
Ian

The ST35 is spec-ed at 17.5W/ch in Push Pull Ultra-linear mode:

http://home.indy.net/~gregdunn/dynaco/c ... index.html

"Sensitivity: 1 volts input for 17.5 watts output (each channel)."


The ST35's OUTPUT TRANSFORMER is Ultra-linear:

http://www.dynakitparts.com/store/schematics/Z-565.pdf


Why matching outputs and speakers (and the 4ohm tap) are important for tube longevity:
http://tubes.nekhbet.com/trans.shtml#spe

[Gingertube]

The most definitive 6BQ5 (EL84) Application Report is here:
http://www.retrovox.com.au/STC6BQ5.pdf

It clearly shows the same as every other data sheet I've seen and confirms the measured result of approximately ten EL84 Ultralinear HiFi Amps I've designed and built and 3 Pentode Mode EL84 Guitar Amps I've built.

Ultralinear Mode 11 Watts
Pentode Mode 17 Watts

My HiFi Amp Design
http://www.diyaudio.com/wiki/Baby_Huey_ ... _amplifier

AND a more sophisticated version which is what I currently use every day (Post #602 and #603 schematics)
http://www.diyaudio.com/forums/tubes-va ... ey-61.html

The Z-565 Output Transformer is rated to 17 watts to give you the option of either mode of operation - Your 12 watt rated transformers will be just fine for Ultralinear Mode.

Cheers,
Ian

[kheper]

The most definitive 6BQ5 (EL84) Application Report is here:
http://www.retrovox.com.au/STC6BQ5.pdf

The UL specs in that sheet have 300V on the plates and the screens, and the output has an 8K input impedance. The ST35 clones on this site show 360V on the plates and 364V on the screens with an 8k input impedance. I have no idea what voltage he wants to run his amp at nor what the input impedance of his tranny is. I am suggesting caution. No more. No less.

The Z-565 Output Transformer is rated to 17 watts to give you the option of either mode of operation - Your 12 watt rated transformers will be just fine for Ultralinear Mode.

Cheers,
Ian

The Z-565 is rated at 17.5W, and UL mode was not optional for the ST35. I am not disputing that the 17.5W/ch rating for the ST35 amp was too generous for marketing purposes (See my first response above), but I suggest that the audio output is higher than 12W.

[bankeny]

Thanks for the help so far guys! Learning a lot!

I got the amp in question and did some quick measurements on the OT.

I'm a rookie here... so correct me if I'm wrong.

I hooked a filament transformer up to the primaries of the OT and measured the voltage on the
8 ohm tap on the secondaries.

It was 7.14VAC in and .268VAC out.
7.14 / .268 gives a turns ratio of 26.64
26.64 squared is 709.69
Multiply that by 8 and I get a primary impedance of 5.68K?

A little lower than the 8K specified... are we still good for an ST35? Or should I look at other circuits?
The PT I think was a 375-0-375, but I will have to look again... Memory isn't serving me too well today!

Thanks again!

Brandon

[Gingertube]

Brandon,
Your maths is correct:
Transformer is 5K678 : 8 Ohms - assuming you connected the source to the 2 anode connection of the primary. That is, because of the lower than the standard 8K Raa value, the transformer is intended for use with lowish rail voltages.
That means that there was probably a tube rectifier used on that 375 - 0 - 375 power tranny.

You can still use it with a pair of EL84, other alternatives are 6V6, 6BM8 (ECL82) and if you can find them the 6GW8 (ECL-86).

It would more exactly suit a pair of EL86 (6CW5).

Impedance matching is important , within fairly slack tolerances, you can tell that for yourself by looking at the impedance profile of any speaker. Nominal Impedance 8 Ohms - probably dips to 5 or 6 Ohms at so low frequencies and will go as high as maybe 40 Ohms at some higher frequencies and at the port tuning frequency on a bass reflex speaker.

So 5 Ohms to 40 Ohms speaker hooked to your 5K7 : 8 Ohms tranny will reflect 3K5 to 28K5 to the primary.
and that same 5 Ohms to 40 Ohms speaker hooked to a more usual 8K : 8 Ohms tranny will reflect 5K to 40K to the primary

You can see why I'm not fussed about the 5K7 Raa in lieu of 8K - it will make very little difference.

Cheers,
Ian

[bankeny]

Thanks Ian,

An update, I measured the B+ voltage (with only 5u4 rectifier tube installed) and was coming in around 380VDC.

Time to order a board and some components I think! Have to sell off some other gear to raise funds for this project.

[Tom Bavis]

That 380V will be quite a bit lower under load.

Your measured transformer impedance is a bit lower than the tubes will actually see, as it doesn't include the effect of winding resistance. Ether measure the ratio again with an 8 Ohm load, or for a quick approximation, measure the DC resistance of the whole primary and double it, and add that to your calculated impedance. You double it on the assumption that the copper loss in the secondary is about the same as the primary.

[kheper]

An update, I measured the B+ voltage (with only 5u4 rectifier tube installed) and was coming in around 380VDC.

Your power supply is probably a 300-0-300. The max voltage should be 425Vdc. The 5U4 drops a max of 44 volts. If I were you, I would try to hit 300-325Vdc. A big choke should drop 4-5% more voltage. If necessary, you can calculate and drop it further with a resistor.