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speakerfritz 30th September 2010 05:08 PM

6l6 tube amp troubleshooring progress
Hi all

I got a Dean Markley 120R tube amp recently. Mainly because it looks a lot like my Dynaco Mark 6's and I want to use it as a center channel along with 4 Mark6's.

Before truning it on, did some basic checks.

Upon inspection, there were mixed matched power tubes, and a replacement transformer installed.

Tested all the tubes, found 1 shorted pre-amp tube, and alhough all power tubes passed, 2 were just two smokey for me to be comfortable with so I replaced them all.

Brought the amp up on a variac, and checked the bias and found them all to be in the 28ma - 31ma range. on the low side, didn't adjust them though, more on that later.

checked around the boards and noticed 2 ( 47R1W) resistors and 2 zener diodes (1N4746) running hot to touch...touchable...but very hot. do I have a power drain here? root cuase of low bais currents?

at full voltage the amp had a background hum.

shut the amp down and checked all the grounds, took a look at the schematic to understand the topology, and began testing. I focused on the section of the power supply around the hot componets. didn't find any shorts, but I found an open 1n4004 diode. I by passed it with a new diode and the hum went away. The amp actually sounded pretty amazing.

so the hot componets. most folks around guitar amps would tell you that hot resistors and diodes are faily common, even to the point where the boards get scorched, as in this case.

the circut involved in the hot componets are responsible for +18 and -18 volts which feed the pre-amp board, and is invloved in the power tube bias circut. I checked the circut and found +- 17 volts. if I got rid of the heat, would the voltage go back up to +-18v?

disconnected the pre-amp board and the componets were still hot, even with no load from the pre-amp circut.

pulled all the power tubes, and the componets were still hot. no bias load, and no preamp board load at this point.

could the zener's have a reverse current leak? probally going to replace them both.

my thoughs are to upgrade the power ratings of the zener diodes and associated resistors so they don't run so hot. The amp works fine now, but im just not comfortable with all the heat on the zener's and resistors.

but as I think about this, i'm wondering would I be transfering the problem upstream. If I beef up the diodes and resistors, would the power trans run hot? Would my tubes no become fuses for the resistors, etc.

also thinking about installing socket savers with low wattage 1 ohm resistors that would fuse out in the event of a future condition that resulted in the tranny being originally replaced. There are no fuses other than the line cord fuse. Whats best Mac had fuses all over the place...grid's, screens, filiments, ps supply, etc.

the schematic is attached. comments and opions welcomed. The test equipment I have avalable to me are inductance meters, capacitance meters, current meters, diode testers, regular multi meter, both emissions and transconductance tube testers, a few variac's, and a 300V/500V tube based power supply.

Basic question, replace the parts with the exact same ratings, or upgrade them. any obvious circut improvements? individual bias? how? where?


speakerfritz 30th September 2010 05:15 PM

1 Attachment(s)

speakerfritz 30th September 2010 05:35 PM

2 Attachment(s)
the bias circut model I'm thinking about can be seen in the following two schematic (a before and after on a nother amp)

firechief 30th September 2010 07:30 PM

I wonder which 1n4004 you found open? one in the main hi voltage section, or one in the + or - supply? I would think that if any of these were open that the amp would have no output. Either one of the 4 in the HV section would render the amp soundless, and either of the ones in the low voltage section would have clobbered the pre-amp. Could it have been shorted instead of open? This is not a big deal, sounds like you are headed in the right direction. The changes/upgraded's you describe are reasonable. As far as the hot parts, a common problem with all older tubes is failing electrolydic caps. The test here is to disconnect electrolydics one at a time and substitute new ones with equivalent ratings (uf and Volts). I would do this with the output tubes not installed. If the voltage comes up a bit and the heat goes away chances are that the cap is on its way out. What power do they claim for that amp. Say 50W? Also I would like to hear more about how you plan to use the bease. Center channel did you say? :cool:

speakerfritz 30th September 2010 08:07 PM

I'll have to look at the board and compare to the schematic to idenetify which was open. It was definetly open (both directions). And when I parallel by passed it, the circut readings were consistent and the hum went away.

The amp is rated at 120 watts.

It's going to be a center channel, on a 5.1 system using 5 tube amps. The other 4 tube amps are dynaco mark 6's re-biased to use kt-88's.

I don't have a problem with replacing caps. It's just as easy to replace them as it is to test them. I think the amp is 15 years old. I'll go ahead and replace the 4 100uf 80v caps in that section of the power supply.


firechief 30th September 2010 08:27 PM

:DLoud Stereo, I like it!

speakerfritz 30th September 2010 11:33 PM

1 Attachment(s)
well parts are on oder for recapping and redioding in the power supply bias area as well as individual bias upgrade. also upgrading the 1 watt hot parts to 5 watts.

the amp sounds crystal's just this heat that bothers me.

I want to do some ome listening and I installed some heat sinks with super glue....dont worry...the brass paper holders are not even close on touching each other.

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