• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dynaco MK III with 6146A tubes.

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An interesting modification out of necissity happened today...

I had my old Mark III out on the bench today after ten years of being moved about and not rebuilt.

It was missing the output tubes, power cap and choke, and some other tidbits.

I finally got the desire to make it work again, except I had no proper parts to fit the original specification.

The best thing on hand? 6146A VHF radio transmitter tubes as outputs, no choke, and bigger caps.

With some rewiring, I used old computer SMPS caps in the PSU, lots in series with resistor dividers to share the voltage drop.

With thi 6146, triode or ultralinear operation requires really really high bias in order to get the plate current down to the necessary level. In my case, about -200V to reach the proper plate current.

Instead of redoing the bias, I removed the U/L configuration and added a regulated 150V supply for the screens. This consists of an OA2 tube with a cascoded pair of SMPS power transistors to handle the necessary 450V drop and boost current.

The new topology sounds very good, with little modification to the actual dynaco circuit.

A picture or two:

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I have just received a pair of Dynaco MK IIIs. But the second tube does not receive any glow. On the original drawing there is NO connecttion to the luggs two and seven! Those are supposed to be connected to the heat filament, are they not?
What am I missing here?
 
don't confuse the output tubes with the rectifier tube. it is pretty easy to do since there is no silkscreen tube markings on the chrome chassis.

The two output tubes are in the middle and the rectifier is on the edge between the ps tranny and can cap.

dynaco_dynakit_mark_iii.jpg
 
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Oh yeah

Stupid rookie mistake from me! Now it's working! Thank you!
Well one of the amps are working, the other is not, but I'll have to wait for my oscilloscope to try see where the signal is lost. 1:st time I have encountered these lovely things, but I'll try to stay away with my fingers from those deadly voltages!
 
I repaired my ST70 for years with a VOM, not a scope. Most problems have a DC voltage cause, and getting the DC within spec fixes the AC. Connect the negative of the meter to chassis with an alligator clip lead so you only use one hand at a time to make measurements.
I have a Simpson 266 100kohm/volt VOM with a 2 VAC scale, which is quite useful on AC. If you put a transistor radio on the input, you can see the beats of rock music with a VOM. Dynaco tube amps have a 1.6 VAC full scale input. You have to put a .047 or .1 uf 600 v cap in series with the input probe to keep a VOM from responding to DC signals. DVM's are okay for DC analysis but they average the reading over several seconds so they are not as useful on AC. If you have an RF oscillation, you can detect it with a VOM by decreasing the size of the blocking cap until the signal disappears, which gives you some idea of the frequency.
Simpson VTVM's are also quite useful. They come with an "RF" probe that has the cap in it preassembled.
 
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The links to the images are broken. A very interesting design. In the 1950s, Acrosound made a 6146 amp with tertiary screen taps.

The 6146 was not noted for being "linear" in pentode mode, but probably most of the amps containing them were cheap and the screens unregulated. The 6146B is supposed to be more linear, and are in current production - should you run out of 6146As.

http://www.triodeel.com/acro6146.gif
 
Robert, you don't need a scope to figure out the problem. first thing I would do is measure dc voltages on all pins and ac voltage on heater pins. you will need to read the tube data sheets to know which are the heater pins.

post results and someone will know what is the problem. tubes not glowing usually means no heater voltages.

if you want, you could swap tubes with the good amp to make sure the tubes are working.

just be careful. high voltages kind of suck.
 
Ok, but...

I only have digital VOM:s, they are great for reading fixed values, not 'beats'. For that an analoge-instrument is needed. I do have a couple of analog instruments, but the only cover up to 5-20 volts (DC. i use them for meauring bias on my tube amplifiers, 2 of them at a time).
So Ill have to wait for my oscilloscope (im used to work with that and a signal generator, not a radio). However that capacitor in series will probably help me alot. Thanks for th tip! I have a 1 microfarad 600V DC 300 V AC capacitor here now. Yes, I will check the voltage before I apply it as the testing probe.
Yes, I will post the results here, for whatever I'll find. I will prbably have to post th scematics of th MK III with measuring points. This should be relatively easy since I now have 2 amplifiers on my workbench, one that is working and the faulty one.
Oh, I almost forgot; both amps have new 6550 tubes and new 'preamp' tubes. The only tube that is original is the rectifier tube, and that one works, I swapped the tube between the ok-amp and the faulty. They did behave as they did.
 
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I would just start with no power applied (make sure supply caps are completely discharged first!) and measure the value of all resistors in circuit, disconnecting wires from the board as needed to get unambiguous readings. Check capacitors for leakage as well. Your dc voltage values are totally off the beam, refer to the tube chart in the construction manual for the expected voltages once you have found and replaced any suspect parts.

Hopefully you have some spare tubes like the 6AN8?
 
I would just start with no power applied (make sure supply caps are completely discharged first!) and measure the value of all resistors in circuit, disconnecting wires from the board as needed to get unambiguous readings. Check capacitors for leakage as well. Your dc voltage values are totally off the beam, refer to the tube chart in the construction manual for the expected voltages once you have found and replaced any suspect parts.

Hopefully you have some spare tubes like the 6AN8?
No problems there, I do have 6AN8:s in spare. I'll do the measurements tomorrow, by then all cps should have reached zero. I'll get back to you. Thank you!
 
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No problems there, I do have 6AN8:s in spare. I'll do the measurements tomorrow, by then all cps should have reached zero. I'll get back to you. Thank you!

Use your meter to check that they are discharged! Do not assume anything!! :hot:
Please read the safety and newbies sticky threads at the top of the tube / valves forum page. Learn how to safely discharge supply capacitors before working on gear...
 
Well that's certainly word of wisdom! Yes, the caps where holding close to 1Volt when I checked. All resistors (but one, the one going to the external plugin where you measure the bias) where up to spec. So, what now? I replaced the preamp tube, but did not test it. Should I?
 
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