Another DG-SE1as a first project

Kkbubar

Member
I’ve been shopping around for a while now for a tube amp I could afford. I noticed that on the auction site that there were these rebuild/repurposed console amps. So having worked in the electrical field doing industrial controls for the last 40ish years of my life I thought maybe I could do this. So shopping shifted from finding a amp to buy to finding one to rebuild.

Enter a console at a local antique shop that “makes noise when plugged in but nothing works”. $20 later I owen a 8600-20, now to the inter web for free information on this unit. This site called AudioKarma has a couple on schematics that match what I have on the bench. A couple of evenings later I stumble on this thread where a gentleman named Dave has walked though my little project amp and not only rebuilt it but redesigned the thing to work much better in today’s electrical environment. And he has explained every change in detail as well as providing a clear schematic of these mods.

Amp on the bench,$150 of stuff in mail and a guy who spent the early years of his career removing old tube based process gear and replacing it with SS. Here we go.
 

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An excellent point by 6dz7. The basic circuit is still an excellent performer. It is also an opportunity to hone your skills at working on an audio amp. I always find that whenever I revisit one of my builds, there are areas that i would have liked to do something different. Maybe a different layout, or route wires or components differently. In a SE amp this makes more of a difference because there is no push pull output transformer to cancel noise.
 
6DZ7 my plan was to take it to bare chassis sands the sockets, so from there it makes more sense to built what I want from there rather than put it back stock as it were. I have a set of AR 14 speakers that this guy is slated to drive. And they are 8 ohm so this route makes the most sense.
 
Got a little tare down done. Those sockets are going to need to go...
 

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I gotta chance to get a little bit more done on my amp today. Finished stripping and cleaning the chassis here's a shot of it ready for devices. Then got all the top side parts mounted. I am really looking forward to seeing it power up.
 

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Quick question I have a 12v led on my power switch I’d like to use. I was going to pull power from the 255 B+ through a voltage divider of 22k and 1k for about 11v @ 11ma, am I robbing to much power from this bus?
 
Got a good bit done on this one throughout this week. Fired up the AC and powered up the DC buses. No smoke , at 120vac I’ve got 279 on B+ and 271 on B++. This is with no load on the PS, seems a little high so I added a thermistor to the in coming AC line. I’ll wait till I finish up the input wiring and bias circuits. The led on the power switch works just dandy...
 

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I got the wiring finished up yesterday and came back tonight to do final checks and bring it up on the dim bulb tester. As I came up though 95vac things were looking good, but as I moved to 100vac I caught a slight arc at the bottom of the rectifier tube and fuse blew. Rechecked everything and tried again same result the second time without the arcing just 1A fuse (slo blo) gone around 97vac. So I’m going to find somewhere to have these tubes tested as I’ve had the PS up once before. But I’ve not test the tubes probably should’ve just got that done before now. I’m going to include a couple of shots of the finished(?) amp and how I’ve set it up for start up. Maybe I’m missing something?

Found a local 2way shop to test my tubes, 6CA4 failed shorted on the second diode. So it seems fine until a load is applied and there ya are shorted. Now off to the interweb for pieces and parts.
 

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Rectifier showed up today and was able to get back out on the bench. As I came up through 70 vac the dim bulb started glowing. Concerned that the bad rectifier had taken a PS cap I took it that apart and ESRed them. All within spec so I put the first stage together and everything checks out. Add in next stage and dim bulb again. Start though everything that’s powered by the B+ and find about 150K to ground on the output transformer primary leads. Found a hair from the OT secondary winding touching the B+ terminal. Put it all back together and bring it up with just the rectifier in and up 122vac no issues. Tubes in 1K on inputs and 8 ohm load on the outputs. At 80 vac we got tone on the left channel scope. Nothing on the right as I go up to 100 vac. Back down to 80 and check voltage and signal levels. RT channel plate is at 105 VDC on the input tube and signals are balanced at the inputs. RT channel has very little signal and it’s the same level all the way though the signal path. Also RT and LF feed back levels are equal. So it’s here that I notice that I didn’t tighten the positive load resistor lead on the jack. Tighten and retry again and everything comes up fine. Check all the levels and everything is okay, voltages are a little low and the bias voltage across the 120 ohm resitor is 5.44 and 5.45 also low.
I decided to hook up the speakers and listen to 1K hz tone, this sounded stable and it was very quite with tone off. Ok music next, sound was muted and there were some cracking sounds, but as it played it became richer in sound and all cracking had stopped. Played it for about 45 minutes and seemed to be getting more full it range. Quit here for the night I’ll go back out and look at square wave and check all the voltages. And report on how it’s doing.

A big thanks to Dave G. for sharing this build. What fun to build a amp from a great design and have it work so well. The only issues I’ve had were self inflicted.
 
Voltages aren’t matching up and the plate voltage on the right channel is quite low. I’ll list what I’ve got out below starting from incoming voltage.

AC line 122 though variac, though the thermistor 119 vac and high windings output is 463 @ rectifier (JJ EZ81)
B+ 247 VDC
B++ 228 VDC
6EU7
RT pin 6 105.4 VDC pin 4 1.34 VDC
LF pin 7 143.1 VDC pin 9 1.28 VDC
6BQ5
RT pin 7 238.2 bias 5.67 VDC
LF pin 7 237.5 bias 5.55 VDC

I believe the RT AF is pulling the B++ down and causing everything to sag that’s related to that bus. I’m also running the led on the power switch from B++ though a divider of 22k and 1k ohms pulling about 11 ma.
Pictures are
Sine wave with .248 input level
Square wave with the same level
And a couple of it running on the bench.

I would appreciate some input on what the next step should be, replace input tube? Remove/jump thermistor? Lose the led supply network?
 

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K -- My development unit does not include a thermistor, so that's part of the issue. The JJ rectifier tube may be part of it as well, but I would suspect that last. The LED is at least pulling the 255 volt source down by 9 vdc, since that is the voltage drop that 11 mA produces through an 820Ω resistor. Also, what is the manufacturing code number for your power transformer? Magnavox used a number of different transformer vendors to keep them fully supplied, but differences can and have been noticed between these parts as supplied by different vendors. For the record, my transformer carries a manufacturer's code of 1386244, indicating it was produced the 44th week of 1962 by Chicago - Stancor.

As for the AF Amplifier stage voltage readings, the voltage at the cathode terminals is largely set by the bleeder current established by the 150K resistors, as the current drawn by the tube itself is typically less than 15% of the bleeder current. This is what allows this stage to effectively operate with fixed bias. If the cathode, bleeder, and plate resistors are all identical between the channels, then all that can be left is a large mismatch between the sections of the tube itself -- which can happen. This was one of the reasons that I changed to a 12AX7, as I have boo-coo of those around, but few if any 6EU7s to check against if these types of issues occur. Check first that the resistors noted are matched between the sections. If so, then I'd try another tube if you have one.

Dave
 
Dave
Thank you for the information, I’ll check on all of that tomorrow when I can get back to the bench.
Kevin
 
Today’s activities, transformer code is 1386223, model # 300206-1 so I think it should be very similar to Dave’s on the development unit.
I removed the thermistor and unhooked the led. Results as follows.
Incoming AC 119
HV output 458
Heater 6.1
Rectifier out 254.7 B+ 250.1 B++ 237.9
6EU7
RT pin 6 109.1 pin 4 1.35
LF pin 7 146.7 pin 9 1.29
6QB5 bias
RT 5.66
LF 5.52
Added led to heater line after checking the switch literature, it is rated 5-12 V ac or dc. Heater voltage unchanged after this was connected.
As this amp is slated to live out here in the shop I’m thinking of removing the bucking connection. I’ve also got some 12AX7’s on the way and will convert to that tube when they arrive. I’ll confirm and match resistors when I do that.

This little amp has really surprised me with how quiet it is and the sound portrayed from the 2way speakers it’s driving. I find it hard to believe that it’s not 100% yet.
 
It’s nice to have good neighbors! Yesterday I was visiting with my neighbor Bill telling him about this amp and he said he thought he had a can of tubes out in his shop. So 4-5 12AX7, 5 6v6, 2 5U4, and a bunch slip shields with other 6 series tubes in them. As we are finishing up the guy to the side of us comes out to say hello. He tells me that the house he just got finished to flip has a old magnavox console stereo in it that he listens to while working and he’s to the point that it needs to go and if I what it just come get it. Don’t know what it is but he said it definitely has tubes in it. Now if I can just get one working...
 
Got a12AX7 wired in today and it seems more balanced voltage wise. Only 4 volts difference between channels but still low on the plates. 110 LF 115 RT. I did check all of the resistors and everything was with in tolerances and matched to a couple of tenthes of a ohm. Power supply came up some but was still 12 volts low on both buses.

So I removed the bucking winding and the buses look much better. B+ 262 and B++ 251, 12AX7 plates are now 117 and 124 and bias on 6QB5 are both 6.23 and 6.24. I do not have the 50 ohm pots installed as they are coming from the Far East on a slow boat. It looks like I’ll be able to set those once they get here. Heater power is 6.51v.

I also went thought the feedback circuits just to be sure there wasn’t something wrong there.

Listened to it for 90 minutes or so and it sounds just great. Voltages are more stable that before they would wander a bit as you played it but it was quite stable today.

I have some 12AX7’s in the mail and I have 3 more from the can-o-tubes as well so I’ll have to try some different ones and see what I get. Please let me know if I have missed anything.
 
Okay I got the 50 ohm pots in tonight and played it for about 45 mins while getting them close, then went to no input and gave them a accurate adjustment. I also tried a different 12AX7 things seem better aligned. On the 12AX7 right plate at 143 and left at 135. PS B+ 267 and B++ 253.

I then listened for another hour even had my wife stop by the bench for some Patsy Cline. This amp is everything I was hoping for. I’m am hooked been looking at the E-linear 6V6 and I’ve got a SE KT88 drawn up with a SRPP front end. Once again thanks to everyone for there advise.
 
A couple of the amp in the final configuration. Transformer has bucking removed and the power supply has a 4 ohm resistor between the choke and the B+. It is setting at 264.8 VDC and B++ 253.7 VDC. A new 12AX7 and a factory matched set of Millard EL84’s. Quite as a church mouse.
 

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