Soundtronics of Chicago

[gutter]

Hello all!

I recently acquired a Soundtronics guitar head from a pawn shop. Very interesting amp. It was built in 1972. It has 3 7025 tubes, a 7199 tube, and 4 8417 tubes. I don't mind the sound but it's so loud that it's hard to break up. And all my research show the 8417 are all NOS or used. I'd like to convert it to something else (6550, KT88) but don't have the slightest clue to what the mods would be. I do like the clean headroom but its almost too much. I played through a 412 cab of the GH12-30.

So any ideas or thoughts would be welcome!!! Thanks!!!

[Structo]

Interesting amp.

Does it hum at all?

You know that the filter caps should be replaced, right?

The 8417 is a tube I'm not familiar with.

It draws 1.6A heater current compared to 1.5A of a EL34.

6550 has 1.6A heater current

So those should work.

I assume fixed bias but I don't see a trimmer pot for adjustment.

If you want it to break up easier you will have to increase the gain on the second preamp tube or use some other trick.

[RandyFire]

Hi,
I recently inherited a Sountronics M-200 Head which sounds sort of like this configuration. It also includes an spring reverb and a footswitch port to control it.

Do you know of anywhere I can get a schematic for it?

For one, what filter caps should be replaced because of the hum? They appear to be just two large cardboard tubes that are epoxied in place. Not sure there is any way to identify them. Also I found a disconnected wire (pink) off of the midrange control and don't know where it goes.

All of the preamp tubes are the standard 12ax that I find in most tube amps.

The 8417's are told to be a "beam focused" tube design and the 6550's, besides being the 1.6A heater current, were told to be that type of design as well.

Any info you have to help would be appreciated.

Regards,

[Stevem]

Since the original output tubes in these amps where 8417s these tubes where specifically designed to run on fairly high plate and screen voltages and they where rated at 35 watts if I recall right.

This would lead me to guess without further voltage measurements from your amp that 6550s will not work for long.

If your amp has weak or dead 8417s then your only option without reporting back on what voltages you have may be to run KT88s, KT90s or KT120 tubes.

Also another thing that looks certain is that to run any other output tube other then a 8417 you will need a lot more negative bias voltage.

This part of the amps power supply may need to be largely modified.

Before you do anything with this amp , like even powering it up please take a whole bunch of very clear gut shots and post them up here.

Also report on what the fuse labeling is listed on the amp and if any of them are blown.

[martin manning]

You will probably need to replace most or all of the electrolytic capacitors to fix the hum and get everything working properly. In the picture above, I see four large can-type capacitors. Since you say it has a hum issue, that must mean it has a set of 8417's in it now. They can be tested to see what condition they are in. They are only available as used or nos, and they are expensive. You can convert the power amp to run 6550 or KT88, the pins and heater current are the same, but the negative bias supply would need to be modified to produce approximately 2x as much negative voltage. Accordingly, 6550/KT88 will require about 2x as much drive signal to get full output power, so the maximum output will probably be reduced.

[10thTx]

I don't mind the sound but it's so loud that it's hard to break up.

IF you are going to convert to another tube anyway for the power tubes, maybe it's worth considering only two power tubes instead of 4?

OR even cathode biased pair of 6V6's with cathode biased pair of 6L6/5881's? You could then pull whichever pair of power tubes you want for numerous tonal options and watts of power.

Just a thought to consider, not saying you "should" do this.

with respect, 10thtx

[imjonwain]

^+1 if you want breakup.

I would push towards not changing it. I understand it from the play-ability and repair-ability stand point but this seems to be a pretty cool and uncommon amp that might be better left as a loud clean platform? I can't imagine there are many of these left or floating around in that good of condition? Just my opinion though.

[Stevem]

Let's look at what these last two replies are proposing for changes.

The change to to four 6L6 or worse yet 6V6 tubes will lower the heater circuit current draw by 2.8 amps, this will kick up the 6.3 volts by enough to shorten the life of all the tubes by a bunch!

The added voltage until tubes do die on you will change the gain structure of the amp due to the higher emissions.

If you remove 2 of the output tubes then your feeing up 3.2 amps of heater current and making things even worse!

I will not even get into the needed output transformer changes!

[10thTx]

The change to to four 6L6 or worse yet 6V6 tubes will lower the heater circuit current draw by 2.8 amps, this will kick up the 6.3 volts by enough to shorten the life of all the tubes by a bunch!

I'm not challenging this as incorrect. I simply have not heard this thought before. Do you have a reference or link that you can provide to support this idea?

I have two amps that are cathode biased & use a tube rectifier, and I've changed between 6V6, 6L6, 5881 and even used 6K6 tubes at around 305v with a 5Y3 ........... so far there has been no issue that I have found in doing this. And I've had one of the amps for over 10 yrs. The heater voltage doesn't seem to change much either as I've measured it. The PT on one amp is 300-0-300 and the PT on the other amp is 275-0-275. I've used 5Y3GT, 5V4, and GZ34 and a solid state plug in with various power tubes.

The idea of using two 6V6's and two 6L6's was from (Londen Power) Kevin O'Connor's The Ultimate Tone series books. He commented it was something he did with Twin Reverbs.

I've read about concerns exceeding a PT's current rating for heater filament doing something like using 6L6's in a Deluxe Reverb for example, but I don't remember reading concerns about using 6V6's in amps that previously had 6L6's when the plate voltage on the 6V6's was not exceeded?

My thinking has been to look at plate and cathode voltages to be within specs ........... and if so, then heater filament voltages should be OK if wall voltages are reasonable?

I was under the impression that a range from around 5.7vac to 6.8vac would work for heater voltage? This may not be correct? I'd love to see a link that adequately answers this question.
I'm very open to having my thinking on this being corrected? Just would like a link to a reliable source that explains this, please.

with respect, 10thtx

[10thTx]

I asked the question about changing power tubes being an issue for heater filament voltage on the Hoffman forum.

https://el34world.com/Forum/index.php?topic=28350.0

If I had a cathode biased push/pull KT66 amp with 400v on the plates and I changed it to using push/pull 6V6 amp with 400v on the plates, would this cause the voltages on the heater filament to raise and lower the life expectancy of the tube? The KT66 draws 1.3A and the 6V6 draws .45A

No, with this caveat: The heater circuit voltage is bound at the upper range by the windings and the primary voltage. On the other hand, if you were to overload the PT heater windings (for example, by using too many tubes with high current draw referencing the spec of the PT), voltage would be lower, but you'd be burning up the PT.

OR if I had a AC30 with 4 EL84's and simply pulled two of them & adjusted the power tube cathode resistor accordingly, would this increase the heater filament voltage and cause a problem?

No problem with the heater circuit in this scenario.

So regarding the comment

The change to to four 6L6 or worse yet 6V6 tubes will lower the heater circuit current draw by 2.8 amps, this will kick up the 6.3 volts by enough to shorten the life of all the tubes by a bunch!

I am not understanding how or why drawing less heater filament current will raise the heater voltage? (if plate and cathode voltages are correct)

with respect, 10thtx

[Stevem]

Here's a question back to you using your line of thinking, if heater current level does not matter then why are the heater windings on any transformer rated for a given amount of current they can provide and then tube spec wise why do there filaments state for example like with a 6L6GC as 6.3 volts and 900 ma?

[10thTx]

So one would not exceed the current draw for heater filaments available from the PT. I am not sure how your question relates to increasing heater filament voltage by using power tubes that draw less current?

Do you have a link to support your previous comment?

With respect, 10thtx

[martin manning]

I am not understanding how or why drawing less heater filament current will raise the heater voltage? (if plate and cathode voltages are correct)

This is mainly due to the resistance of the heater winding wire. The voltage increases by 7-11% from fully loaded to unloaded conditions (according to the Hammond transformer data sheets I checked), and the voltage vs. load is going to be fairly linear.

[10thTx]

This is mainly due to the resistance of the heater winding wire. The voltage increases by 7-11% from fully loaded to unloaded conditions (according to the Hammond transformer data sheets I checked), and the voltage vs. load is going to be fairly linear.

Yes, I agree. Unloaded has higher voltage on the heater filament voltage. Nodes on the B+ will show higher voltage unloaded than when loaded also. I am aware of that.

My question is "Would it be problematic for heater voltage with something like a four 6L6 amp to run two 6L6's or four 6V6's? Is that going to raise the heater voltage in a manner that hurts the tubes?"

So not loaded vs. unloaded (which obviously would have a significant current draw difference) but instead loaded with 6L6 vs loaded with 6V6 (which would have less of a significant difference in current draw than loaded vs. unloaded)

I have not been aware of that being an issue? Not saying it wouldn't be, it's just that I've never heard of that before?

Respectfully, 10thtx

[sluckey]

My question is "Would it be problematic for heater voltage with something like a four 6L6 amp to run two 6L6's or four 6V6's? Is that going to raise the heater voltage in a manner that hurts the tubes?"

no