Revival of 2 Dynaco Mark II amps

Ok, here is my situation.  When going through his grandmother's house, a friend came across two, Dynaco Mark IIs that belonged to his uncle.  His uncle didn't have any interest in them and told him that he could have them and only asked for a cut if he sold them.  My friend has no knowledge of electronics, so he asked me if I could bring them back to life.  I've built and modified guitar stomp boxes and made minor repairs on solid state guitar amps, but never worked on tube amps (to be honest I'm a little intimidated by the high voltage).

I was really glad when I found this forum and I'm hoping to get some advice on how to proceed.  A quick scan of the site shows only a few posts specifically on the Mark IIs so I'm wondering if they are generally less desirable than the Mark IIIs or IVs?

Since I think my friend wants to sell the amps when they are in working order I was thinking that keeping them in as close to their original condition as possible would be best.  My plan is to test the components and only replace things that are clearly not working.  Also, the chassis is not shiny like some of the pictures of the Mark IIIs I've seen.  Was this the way the Mark IIs were made or has it just become dull over time?

Finally, I'm interested in getting a little more information on these amps in general, so any information of this type would also be greatly appreciated.

-- Athena

The Mark II is essentially the same as the Mark III.  It is slightly lower in power: it is a 50 watt amplifier; the Mark III is a 60 watt amplifier.

It has a simpler power supply...a 5U4 rectifier tube instead of a 5AR4, and instead of the filter choke it uses a big power resistor.  It thus has a lower B+ voltage.  It has EL34 output tubes instead of the 6550s in the Mark III.

The output transformer on the standard Mark II had only 8Ω and 16Ω taps but is otherwise identical.  The Mark II was available with the Mark III output transformer, having 4Ω, 8Ω and 16Ω, at additional cost.

The power transformer of the Mark II is identical to that of the Mark III.

The driver circuit, which is entirely contained on one circuit board, is the identical pentode-triode circuit, using a 6AN8 tube, as the Mark III.

Servicing a Mark II is identical to servicing a Mark III.  Setting the idle current (commonly called "setting bias" is done the same way; the different idle current requirement of the EL34 tube is accounted for by a cathode resistor of a different value (resistance).

Several points in such an old Mark II will likely need work: the output tubes may be tired (the 6AN8 can last forever).  The quad power supply filter capacitor (the same one sold for the Mark III at Dynakit Parts is used in the Mark II).  The selenium rectifier in the bias supply may have gone high-resistance; if so, the symptom being inability to bring the voltage measured during bias adjustment low enough, a modern silicon diode should be soldered across it (OBSERVE POLARITY).

The chassis of the Mark II was plated in dull silvery metal; the Mark III in chrome, which you noticed.  The cover of the Mark II was black; most Mark III were brown; and the location of the fixing screws was different between the two.

GOOD LUCK!

GP49 - You have provided one of the most concise and informative responses to a query on this forum that I've ever read. Kudos.

Thanks GP49, this was very helpful.

On one of the units, the quad capacitor had leaked electrolyte so I'm sure that one will need replacing.  For the selenium rectifier, can I test if it is functioning properly using the diode check function on my meter?  What should I expect for readings in each direction?  Ideally, I'd like to find all the problem components and make a single parts order, or is that hopelessly naive and over optimistic when working on electronics of this vintage?

One other question.  I haven't tried to plug either unit in and turn on yet, but when I do, do they need to be connected to a load (speaker)?  Both of my tube guitar amps caution against powering up without a load connected, so would I be correct to assume the same would apply here?

-- Athena

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Here is a suggested powering up sequence:

Start with either no speaker connected or with a dummy load or "sacrificial" speaker connected. Rotate the bias potentiometer fully left and fully right several times then center it.

1) Pull all the tubes before powering up
2) Insert a 60-100W incandescent "dim bulb" in series with the fused leg of the power cord
3) Plug the amp under test into your variac or directly into an outlet
4) Observe the lamp as you raise the variac output or plug in the amp

The lamp should be dark or maybe glow dimly for a moment then go dark. This is good. If the lamp glows continuously the relative brightness will tell you how much current is flowing. With no tubes in place this should be near-zero leakage.

Repeat the process with all tubes inserted except the rectifier (could be 5Y3, 5U4, 5AR4) inserted. Repeat steps 1-4. Now the bulb should light up at power up due to filament current. The lamp should visibly dim to a steady state after 5-20 seconds. This is good. If the bulb stays bright you have some debug to do. There's help here.

For the last step the output load or speaker needs to be connected. Insert the rectifier so all tubes are in place. Repeat steps 1-4. The lamp should glow brightly at power on then dim to a lower level after 5-20 seconds. All filaments lit? No dull cherry red glow in the output tubes? This is good. If the lamp stays bright or more than the filaments in the output tubes are glowing you have some debug to do. If all looks good at this stage you can apply a line level signal to the input and see if there is any output. The bias level will be wrong due to series resistance of the lamp. This is expected. If you hear undistorted output at low or moderate volume levels power the amp down. Take the "dim bulb" out of the circuit and test the amp under normal operating conditions. Now you can set the bias per the MkII manual.

Since you have visible issues with a quad cap you can expect some trouble along the way, e.g. fuse blows, output hum, transformer buzz, no output, etc. Your options are to replace the quad cap with a similar cap ($$$) or bypass the quad cap and install individual electrolytics from your local guitar amp tech or your favorite supplier "below deck".

there were at least two versions of the MK2, the early ones had a square PCB, the later ones used the same exact board as MK3's
the manual for the later version is available on my website here:
http://tubes4hifi.com/downloads.htm

Athena, great help here; and welcome to our group. Keep output/input loads on the amps. Be certain to read the STICKY here on SAFETY .
And I would stick with Tubes4hifi on all my parts. Stay with us. Dale

Wow!  Thanks for all the suggestions and encouragement.  I'll definitely keep the updates coming as I dig in, as I'm sure I'll have more questions as well.

It looks like I've got the later version, as the circuit board is not square.  Is there any way to narrow down the date of manufacture on the units?  My next step will be to use the documentation tubes4hifi provided and start verifying all the connections.

-- Athena

Well after tracing out the circiut and comparing to the schematic, the two units appear to be in pretty good shape.  The only obviously broken parts were the quad caps, the rectifier tubes (open circuit on filament) and the power switch on one unit.

So far I've replaced the quad cap, rectifier tube, and switch on the first one.  I powered it up gently with a Variac, and all the tubes glow nicely.  Initially it popped a bit - infrequent random intervals, becoming less frequent as the amp was left on.  After about 15 minutes that has stopped, but it does come back each time I turn the amp on cold.  Any recommendations on what to check for this?  I'm hoping it's not the power tubes, since they are old Dynaco labeled ones that probably came with the unit when it was new.  To test the sound I have the input connected to the headphone jack of an old iPad.  The sound is very clean - no hum or crackle, at least after the popping stopped.

.

After a tube the next possibility is a defective capacitor that arcs over under working voltage. Those on the board, if original, would be at least 50 years old or so. A defective quad cap is possible too, even if new. Production glitches do happen

even if you decide you want to keep these stock, I would at a minimum replace ALL capacitors and all resistors, they are 60 years old.
Probably $30-40 worth total, not including the two quad caps. Or transform these into modern state-of-the-art amplifiers for around $100 in parts
(plus the cost of 4 new driver tubes) with our VTA MK3 replacement PCB kit.

OK, both amps are now up and running and only required a small adjustment to bias the tubes.  The second amp checked out OK and sounds great after getting a new rectifier and quad cap.  It looks like the initial popping on the first one was the sockets.  After a little cleaning the popping is gone.  But since the power tubes are original I suppose it's anyones guess how much life they have left in them.

I realize replacing all the capacitors and resistors will ensure the amp will run trouble free for years to come, and if this amp was for me to keep I would definitely do that.  Since I've been fixing this for a friend who is interested in selling it, my assumption was that a likely buyer would be more interested in correctly working units that are in as original condition as possible.  I guess I don't really know the market on vintage audio equipment - is this a good assumption?

.

Athena_s wrote:OK, both amps are now up and running and only required a small adjustment to bias the tubes.  The second amp checked out OK and sounds great after getting a new rectifier and quad cap.  It looks like the initial popping on the first one was the sockets.  After a little cleaning the popping is gone.  But since the power tubes are original I suppose it's anyones guess how much life they have left in them.

I realize replacing all the capacitors and resistors will ensure the amp will run trouble free for years to come, and if this amp was for me to keep I would definitely do that.  Since I've been fixing this for a friend who is interested in selling it, my assumption was that a likely buyer would be more interested in correctly working units that are in as original condition as possible.  I guess I don't really know the market on vintage audio equipment - is this a good assumption?

Yes, an unmunged amp is more attractive then one where a number of unknowns are
introduced. To the list of unknowns one might say :
- mysterious caps , maybe with wrong / changed values
- different brands of resistors, again maybe wrong properties / values
- soldering done with unknown quality and unknown solder ( all sorts of intermittent problems)
- broken / damaged connectors, cut and spliced transformer leads
And the real killer is :
- undocumented parts and / or modifications
- replaced "vital" parts from unknown sources

Yes, you are wise to only replace the needed parts, next owner will thank you,and your friend
will be able to get a maximum price.

After thinking about what to do with the amps for a couple of months, my friend has decided to sell the pair.  I'll put a link in the appropriate section for his eBay listing.

Thanks again for all the good advice.