DR405 filter caps and repairs

[synthi]

Thanks for posting that link Zells, I am definitely getting more and more excited about having the amp going in full song soon. The parts order is about to be sent off and I also just snagged a couple late production NOS Mullard CV4024s for a reasonable price(!), one of which will be ending up in the hiwatt as the PI of course.

Further on the topic of NOS tubes, has anyone got an opinion on the early 60s Sylvania grey long-plate 12AX7? I expect it might be OK in this amp... figure with the long plates it should have plenty of gain and a fairly late break-up, which for a bass-amp sounds just like what i want for V1 (and certainly couldn't hurt for the driver either). I'm not a maniac for NOS tubes, so don't want to get crazy here, but just figure it might be nice to have a couple of options to try.

Also wondering if anyone's measure the output of these beasts? Do they actually get close to the 400W mark? I always figured it would be a bit of a stretch and especially with current KT88s.

[menace2u]

Also wondering if anyone's measure the output of these beasts? Do they actually get close to the 400W mark?

At last years VHC I saw Zells evacuate an entire building when he dimmed his 405 and plugged in a guitar.

I was chatting with the good Dr. at the time and after we got outside (AND CLOSED THE DOORS) we still had to move away from the building to continue the conversation!

It was at that moment I knew some day I would have to build a 405 for myself!

[Dr.HI-TONE]

400w. I don't know.

But, "Earth shaking", "mind numbing" and "shorts soiling" Loud...............Indeed!

[synthi]

But, "Earth shaking", "mind numbing" and "shorts soiling" Loud...............Indeed!

<mr burns voice>
eeeexcellennntttt!!!
</mr burns voice>

That's pretty much what I'm going for with this thing. One of the cabs is an 18" PA sub!

[Zells]

Thanks for posting that link Zells

You're welcome.

Further on the topic of NOS tubes, has anyone got an opinion on the early 60s Sylvania grey long-plate 12AX7? I expect it might be OK in this amp... figure with the long plates it should have plenty of gain and a fairly late break-up, which for a bass-amp sounds just like what i want for V1 (and certainly couldn't hurt for the driver either). I'm not a maniac for NOS tubes, so don't want to get crazy here, but just figure it might be nice to have a couple of options to try.

Long plates vs. short plates doesn't make a noticeable difference in gain. Vintage Sylvania tubes are good. My favorite 12AX7/ECC83 tubes are the Mullard ECC83 long plate D-getter and the Telefunken long smooth gray plate. However, Dr.HIWATT has previously said that short plate tubes tend to be more durable since the lower weight short plates do not stress the welded supports as much.

Also wondering if anyone's measure the output of these beasts? Do they actually get close to the 400W mark? I always figured it would be a bit of a stretch and especially with current KT88s.

Someone reported results on the 'net (I forget which site) from bench testing a vintage DR405: 445 watts clean with <3% distortion.

At last years VHC I saw Zells evacuate an entire building when he dimmed his 405 and plugged in a guitar.

I was chatting with the good Dr. at the time and after we got outside (AND CLOSED THE DOORS) we still had to move away from the building to continue the conversation!

I was using a single 4x12 cab loaded with a quad of the HITONE 150w DR-F '142 prototype speakers. You should hear the DR405 through a quad of 4x12s. Playing bass through a quad of Eminence Kappalite 3015LF speakers (which are PA speakers) is an intense experience as well.

I rent studio space in an old converted warehouse. The other musicians in the building know that Zells is the loudest.

[mhuss]

I rent studio space in an old converted warehouse. The other musicians in the building know that Zells is the loudest.

Metal Band Member 1: WHAT SONG ARE WE PLAYING?
Metal Band Member 2: I DON'T KNOW, ALL I CAN HEAR IS THAT &@*#$^ ZELLS NEXT DOOR!



--mark

[synthi]

So, after a bit of a hiatus, we're finally back on the case.

Yesterday the filter caps were all replaced and today the screen-grid resistors. All is looking good (many many photos to come), however we've noticed something ODD on the OT secondary wiring to the impedance selector. Looks like my transformer doesn't have a 16 ohm tap(!?).

There is red wiring (3) connected to the 4 ohm position, yellow wiring (2) connected to the 8 ohm position which is then linked to the 16 ohm and 100v positions on the selector. The -ve feedback wire (white) is connected to the 16 ohm position on the selector. This seems rather strange to me. There are no other wires emerging from the OT secondary as far as I can see. There's one Black (common), 3 Red and 2 Yellow, connected as I just described above. The wiring is still tightly loomed and covered in red paint, so it looks to be factory original. Anyone know whether this is 'normal' for a DR405. My OT is a model TH1384 as described on The Prof's schematic.

impedance_selector.jpg (181.39 KiB) Viewed 2552 times

Also, it was wired with the KT88 DR201 style output jacks and someone has removed the 2nd shorting link so that there is no need to use a 2nd plug when only using a single cab. I'm OK with this being the way it is, but would kind of like the series-parallel combo instead as I have 2 8 ohm cabs that I plan to use normally, but another 2 8 ohm cabs that could also be used with them on occasions.

Anyway, here's a picture of the wiring, which looks stock apart from the fact that the first cliff jack has been replaced with a black one and the removal of the diagonal link between jacks 3 and 4. If I don't re-wire it for series-parallel, is there any reason to replace that link? Frankly I can't see one. It would just mean I need the extra jack if only using one cab and increases the risk of the OT being shorted... Am I missing something here?

output_sockets.jpg (194.07 KiB) Viewed 2552 times

Cheers,
-Matt.

[synthi]

Initial smoke test passed with flying colours!
What a relief

All valves removed, brought up on the variac to ensure nothing gets over-volted.

Linevoltage: 240VAC (from Variac)
Voltage selector: 245V

HT1: 707 VDC
HT2/3: 413 VDC
BIAS: -141 VDC
32uF: 405 VDC
16uF: 402 VDC

That might do for today, but it sure is tempting to load it up and see.

[synthi]

danger will robinson!.jpg (201.42 KiB) Viewed 2549 times

[mhuss]

Yes, 700+ volts DC commands respect. That could easily kill you if you touched it while touching the chassis with your other hand.

A lot of the bilfilar Partridges only had two sets of secondaries, many 504s are this way. I'd be curious for the other 405 owners to chime in with what their OT secondaries look like. The few 405 pictures on Hiwatt.org look like they may have the same wiring (4 wires, 2 colors).

--mark

[Ted B]

Yes, 700+ volts DC commands respect. That could easily kill you if you touched it while touching the chassis with your other hand.

Fortunately, becoming inadvertently familiar with a DR201 in this manner just left me momentarily shell shocked and with some residual soreness. I don't recommend it.

[synthi]

Mark, that's some good info about the OT. I wasn't really concerned about it given that the amp is designed to drive at least 2 cabs; the lack of a 16 ohm tap isn't really an issue. All four cabs that I have access to are 8 ohm as well, so again not a problem. This still leaves me keen to re-wire the sockets for the series-parallel combo though (for 8||8 in series with 8||8 for 8 ohm total). Realistically though, I should probably wire up my own series box and do that externally rather than mess with the amp unnecessarily.

We were very cautious with the open amp powered up (and my place is fitted with earth leakage detection safety switches on the mains).

Next step is to clean up all the valve sockets and then check all the voltages at each of the pins to ensure everything is A-OK before fitting the tubes. I expect with that kind of B+, ensuring the sockets are in good shape will be rather important. To clean them, I normally just use an old tube with the pins wet with contact cleaner and insert and remove it a few times. Does anyone have any other suggestions?

Also, regarding bias, I don't really have enough dummy load (max 400W at 8 ohms) at hand to do a full power test, so will probably just set the idle plate dissipation to a reasonable value, say around 28W and check that all is looking good on the scope when driven fairly hard, but within the envelope of the load.

We have a bias probe, so I will probably check that each tube is reasonably well matched and then compare the value to that calculated from the OT voltage drop. I measured the transformer yesterday with the fluke calibrated on the low ohms range and got a respectably matched 20.4 and 21.9 ohm for each half of the primary.

Mark (or anyone else for that matter), do you have any suggestions for biasing the amp, or does that sound reasonable?

Once it's all up and running nicely, I plan to let it blast 100HZ at 1-200W or so into the dummy load for a good couple of days to soak the tubes and then recheck the bias.

Hopefully the pots are still good - I plan to check them first before giving them a squirt.

Fortunately, the original compliment of Mullard pre-amp tubes still look really good and clean, with plenty of flashing left. I've also got a NOS Mullard CV4024 and a full array of new Tungsol 12AX7s to compare them with and will keep whichever combination sounds better. The remaining 4 GEC KT88s look pretty burned up with lots of oxide/soot(?) on the plates.

Once my new broadband service is turned on at home, I'll upload all the photos to flickr and add the beast to the registry on hiwatt.org.

Cheers,
-Matt.

[synthi]

p.s. Ted: Ouch! glad to hear there was no permanent harm done!

[mhuss]

The "old tube and contact cleaner" is probably the best method.

I'm not sure which biasing method you are asking about. I would still bias them at 70% dissipation or so.

--mark

[synthi]

Thanks again Mark.
Re: biasing, I was planning to aim for the ~70% idle dissipation target since I can't test it under full load (unless I go buy another set of 8ohm 100W dummy loads ).
In cases where I can run an amp to the onset of output stage clipping, I find looking for the cross-over notch on the scope to be a good biasing option, but followed by a sanity check the bias by measuring the plate dissipation is at idle to make sure the value is reasonable.

Regarding the output transformer voltage drop method, provided you can measure the DC resistance of each half of the OT accurately, I find it works quite well. In short, I measure the voltage across each half of the OT, divide by the corresponding resistance, then again by the number of tubes in that half of the amp to work out the idle current. From there, I then multiply by the measured B+ voltage to get the dissipation. Obviously this relies on relatively well matched tubes for any degree accuracy.

With any luck it'll be up and running this coming weekend