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D150 monster amp and funny tooobz
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Old 28th August 2019, 01:06 PM   #61
koja is offline koja  Canada
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I take it then that my hand marking is correct? bringing it in after the C3 and then just finding the place for 39pF filtering to add.

On the subject of which resistors may have been at risk when V19 threw fireworks: R34 and the bias pot may also have to be checked?

FYI today I am ordering a matched pair of Sovtek 6550WE for screen regulators and will be moving the original ones to OP V15/V19 after checking the suspect resistors.
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Old 28th August 2019, 04:51 PM   #62
rayma is offline rayma  United States
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Quote:
Originally Posted by koja View Post
I take it then that my hand marking is correct? bringing it in after the C3 and then just finding the place for 39pF filtering to add.

On the subject of which resistors may have been at risk when V19 threw fireworks: R34 and the bias pot may also have to be checked?
Foe balanced inputs, if you want an input filter, you need an RC filter on each of the input lines
(both pins 2 and 3) to balance the impedances.

You can check R34 along with the rest. Personally I would check all of the resistors and diodes
while the amplifier is opened up.

The input coupling capacitor C3 is best removed. If you want to keep it in the balanced version,
you need one for each line (both pins 2 and 3).

Last edited by rayma; 28th August 2019 at 04:54 PM.
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Old 28th August 2019, 05:28 PM   #63
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by rayma View Post
Faux balanced inputs, if you want an input filter, № 1 - you need an RC filter on each of the input lines (both pins 2 and 3) to balance the impedances.

№ 2 - You can check R34 along with the rest. Personally I would check all of the resistors and diodes while the amplifier is opened up.

№ 3 - The input coupling capacitor C₃ is best removed. If you want to keep it in the balanced version, you need one for each line (both pins 2 and 3).
№ 1 - I agree with.
№ 2 - R34? Look as I might at the schematic, no R34.
№ 3 - C3 removal…

Reading a prior comment, wherein you regard C3 as a high capacitive load on the source, I beg to differ:

First, the 10 kΩ resistor from RCA input to pot limits whatever impedance load to just that.

Second, the 'knee' in the F=1/(2πZC) relationship is 1/(6.28 × (470,000 || 301,000) × 0.47×10⁻⁶) → 1.5 Hz. That's with the pot wide open.

Third, the right-hand-side of the 0.47 µF isn't connected to ground, effectively or physically, sooo … the idea of it being a significant capacitive load kind of vanishes, you know?

Anyway… just a spurious comment, I suppose.

Just saying,
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Old 28th August 2019, 06:07 PM   #64
Tubelab_com is offline Tubelab_com  United States
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Quote:
resistors may have been at risk when V19 threw fireworks: R34 and the bias pot may also have to be checked?......R34? Look as I might at the schematic, no R34.
First place I looked, the grid to bias supply resistor on V19.

I would fire this amp up with no output tubes installed and verify that the plate, screen and bias voltages are somewhat close to what's expected. Yes the voltages will be a bit too high, so I would use a Variac set to somewhere around 100 volts if one is available.

I was experimenting with a DIY amp that makes 75 watts per channel from pairs of 6550's when there was a white flash and a blown 6550. The plate supply had died (I knocked the plug out) while the screen grid remained powered. The poor grid attempted to eat all of the plate current, and was vaporized.
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Old 28th August 2019, 07:42 PM   #65
rayma is offline rayma  United States
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Quote:
Originally Posted by GoatGuy View Post
Reading a prior comment, wherein you regard C3 as a high capacitive load on the source, I beg to differ: First, the 10 kΩ resistor from RCA input to pot limits whatever impedance load to just that.
We're not talking about using the RCA jack input, but rather adding an XLR connector.
If he leaves C3 installed when going balanced (taking the positive input phase to R12),
then C3 will hang off the positive phase directly to ground when the (unused) pot is at minimum.
Not good. Also C3 degrades the sound so badly that it should be removed, regardless.
I've removed C3 for several D-150 owners and all were very happy with the results.

Last edited by rayma; 28th August 2019 at 07:54 PM.
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Old 2nd September 2019, 10:25 PM   #66
koja is offline koja  Canada
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opened up the amp and got the pcb in the middle exposed. measured everything. 10 Ohm/3W dale resistors by V19 and V15 were done in. Will order replacements this week. The bias pots seem to check out too, phew. The screen regulator tubes are in the mail . So hopefully in a week I can get the amp back in shape .

p.s. re: balanced input : I see now the symmetry R7/R8 so I got your point re: keeping R7 in the circuit. Will have to check the board layout and how best to do that. Thx.
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Last edited by koja; 2nd September 2019 at 10:29 PM.
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Old 2nd September 2019, 10:30 PM   #67
rayma is offline rayma  United States
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You want the input grids to always have a ground reference, whether or not there is an input connected.
That's what the two 470k resistors do. The pcbs are very delicate, use extreme caution when desoldering.

Check all the rest of the resistors on the board while you can, because it's easy to do right now.
Also clean the pots with good contact cleaner now.

Last edited by rayma; 2nd September 2019 at 10:52 PM.
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Old 11th September 2019, 04:03 PM   #68
koja is offline koja  Canada
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I received a pair of Sovtek 6550WEs from Jim (who was recommended earlier in the thread). However these are only loosely matched compared to the original AR 6550As. They will probably be good enough for the grid regulator. However this puts a new light on how valuable the original tubes are, which based on hand written values on the stickers have been matched to 2 digits? AR must have had a large stock from the same batch to come up with such close matches.

So now I am thinking of the safest way to bring those online . My friend is suggesting that I run them first with plate and screen fuses removed for 24 hrs to allow for some degassing (since these tubes probably sat unused for a very long time). Then use a variac to slowly bring the anode voltage up allowing for some heat up time, but doing a pair at a time may take a long time to complete? Any thoughts on what a reasonable course of action might be?

Last edited by koja; 11th September 2019 at 04:07 PM.
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Old 11th September 2019, 04:19 PM   #69
rayma is offline rayma  United States
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I would not run the tubes with the fuses removed. Instead, turn the amp on with the power switch
in the 240V setting, and leave it there for an hour or two. That should be good enough.

Last edited by rayma; 11th September 2019 at 04:21 PM.
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Old 11th September 2019, 04:55 PM   #70
H713 is offline H713  United States
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Depends on how long the tubes have sat. With WWII surplus tubes, I like to run just the filament (no grid, screen or plate voltage) for 24 hours to clean up the vacuum- helps keep them from arcing. I might be tempted to create a jig to do this though, as some amps really don't like running without load on the B+. These tubes probably haven't really sat for that long, so 24 hours is probably excessive.

That said, it sounds like you already had one of the original tubes arc, so maybe they are a little gassy and a bit more burn-in time wouldn't hurt.
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