Dumbleland Special 150w ser#009 Schematic

[HaenZ]

You guys might be interested in Dumbleland #011, schematic attached to this post.

[martin manning]

Very interesting indeed, thanks for posting this link!
Comparing the step filters with what has been posted here before and the revised filters I proposed, I see the bass control resistors are simulating a W taper, which I also found evened out the steps. On the treble side it looks similar to 002 SSS, except with more steps and two ranges. I'll have to study this some more, but I think I'm still partial to the revised version that has even steps on the treble side and balanced treble bass effect. It also shows conventional pp output with 6550's. Too bad there aren't more voltages on the schematic, but with 800V CT on the PT, the plate voltage is high, shown at 500V+. I'd think that would not play well with the screen voltage close to plates (only 500R dropping resistor from B+1 to B+2. Cathode follower driven PI is unique, as is the variable GNFB.

[martin manning]

PS, Did you happen to measure the OT primary impedance?

[HaenZ]

PS, Did you happen to measure the OT primary impedance?

Unfortunatly not. If there is more to investigate I might open it up once again.
Voltages can be added then aswell.

[martin manning]

If there is more to investigate I might open it up once again.
Voltages can be added then aswell.

Thanks! You don't need to open it up to measure the OT. You can feed signal into the speaker jack and measure across that and the output tube plate pins to get voltage ratio. The figure you have for the B+1 voltage is enough, really.

[martin manning]

Note, if you measure the OT in place, use a low voltage at the speaker jack, you will be putting AC on the filters through the CT, so keep it low. I would use ~100 mV.

[neskor]

there is one right now on ebay.de for sale
https://www.ebay.de/itm/158044174639

[didit]

PS, Did you happen to measure the OT primary impedance?

Slightly better than just a guess, the output transformer was either pulled from, or simply a repair part, from Sunn and was designed cloning the spec's if not the deep winding detail of a Dynaco a451-- so 2.2K UL ..

[martin manning]

But evidently not wired for DL operation. 2k2 would be high for 4x 6550, putting the screens at risk. Exactly how HAD was operating the power stage in the 150W amps has been a mystery (at least to me) for some years.

[didit]

Wondering if you've done the model as it is captured in the schematic. GE's 6550-A variant have a reputation for ruggedness. The 10W transient screen dissipation is not nothing. All that however, and my rough calculations put the screen voltage up around 460-475, and plates up in the 490V range. This swag is doubtless a bit sloppy in predicting where the screen supply node lands. Would have been good to have had accurate measurements across the all the nodes of the power chain.

Interpreting with interpolation GE's datasheet guidance for "fixed" AB1 P-P pair suggests primary of roughly 4.2K for plate voltage in that range but also calls for ~300V on screens. 2.2 is ballpark correct for a quad -- and does yield 150W on paper (metaphoric).

FWIW ..

[martin manning]

I haven't modeled it with a proper simulator, but with those numbers it would look like this.

[didit]

The extreme technical term for those slopes is -- the hairy threshold and beyond ..

[dbharris]

You guys might be interested in Dumbleland #011, schematic attached to this post.

Thank you so very much for sharing! I wish more shops documented the Dumble amps coming through their store like this. I hope the Dumble Preservation Society does not take any issue with this, though they would have no legal recourse of course.

Besides seeing all of the actual values the most interesting thing for me is the cathode follower stage driving the Master volume. I wonder if this stage is in all of the DLs?

-Dan

[martin manning]

I second that! It’s been a minute since we have seen new information of this quality. The CF driven PI is new to me too, and I don’t think I’ve ever seen that before. Seeing another implementation of the filters has been very enlightening too, and I think I understand HAD’s intent much better now, and my questions on the 002 hand drawn schematic have been largely cleared up.

[martin manning]

If there is more to investigate I might open it up once again.
Voltages can be added then aswell.

Thanks! You don't need to open it up to measure the OT. You can feed signal into the speaker jack and measure across that and the output tube plate pins to get voltage ratio. The figure you have for the B+1 voltage is enough, really.

Note, if you measure the OT in place, use a low voltage at the speaker jack, you will be putting AC on the filters through the CT, so keep it low. I would use ~100 mV.

HaenZ, just to be clear, there is no risk of doing any damage here, the AC voltage appearing on the filter will be very very small.