Mystery clone

[Lynxtrap]

Notice anything odd?

It's a Dumblelator
A few things quick. the 22k RC filter network on the grid of your recovery amp is suppose to be 220K/500pf
on the schematic it's marked 22k but the layout has 220k.
The unmarked bypass cap for recovery (2.7K) says 3uf on the schematic and is marked ? on the layout.
the original is 47uf.


The 2 things that convince me that it's a Dumble design are 220k/500pf grid network and the NFB loop around the recovery amp is classic Dumble trademarks.
BTW the 10k on the cathode of the C.F is used to set the voltage constant at around 30V for optimal frequency response/headroom. Like Rootz says this value can vary depending on the voltages from the P.S
Tony

Thanks!
22K on the grid was a typo when I did the schematic, it is 220K. The cap value is not visible but I guess 500pF would be a safe assumption then.
The only marking visible on the cathode bypass cap is ”3.J” so 3 uF was just a guess, so I put a question mark after the value in the schematic. The neighbouring cathode bypass cap is identical. Haven’t seen such a short code on a cap before. J is supposedly the tolerance. The underside can’t be seen without unsoldering it, there might be more labeling there.

[rootz]

Tony, is the dumbleator in the files section Robben Ford’s actual one? That one does seem to have a 27k tail on the CF and a 1k8 bias resistor. Is that setup considered to be the standard?

[Charlie Wilson]

Don't think "standard" applies to Dumbleators. Lnxtrap, your layout is more of a High Plate Classic with some tweeked values than a Skyline.
CW

[talbany]

Tony, is the dumbleator in the files section Robben Ford’s actual one? That one does seem to have a 27k tail on the CF and a 1k8 bias resistor. Is that setup considered to be the standard?

Thats a double no There are a few out there that have been known to have different value tails AFAIK
This one also has a 10k send pot
Tony

DumblelatorIISchematicUpdate.jpeg

[Lynxtrap]

All these switches fry my brain.. But sorting through some schematics that were sent to me, I think the basic topology is actually the one called ”Hybrid Amp Rev A”. At least close enough that I can copy that schematic as a base and make changes where it differs from the actual amp.

Searching on the forum I found mentions of that circuit here and there, but what is it really?

[talbany]

All these switches fry my brain.. But sorting through some schematics that were sent to me, I think the basic topology is actually the one called ”Hybrid Amp Rev A”. At least close enough that I can copy that schematic as a base and make changes where it differs from the actual amp.

Searching on the forum I found mentions of that circuit here and there, but what is it really?

The "Hybrid A" is one of the first schematics of an ODS posted on the web IIRC early 2000's There were several versions added as more info leaked out and 124 was later exposed and blueprinted here. Some of these Hybrid designs were more or less versions of both high plate (see below) and low plate. Some things were missing and some had incorrect part values, however the topology was basically the same as an original 4th generation mid 80's Classic w/ Deep 80's ODS. I cannot remember who drew them or what amp they were reversed from?
Here is a Low plate Classic from the same era. https://ampgarage.com/forum/viewtopic.p ... 62#p140262

Tony

hybrid_a.gif

[Lynxtrap]

Well, this should be it then. As can be seen, it is pretty close to the "Hybrid Amp Rev A" with the major difference being the added D-lator.
Anything out of the ordinary that sticks out?
The Presence pot seems a bit funny to me.

Next step is to put some glass in it, power up and see what happens

Mad Cat Special 40 schematic.pdf

[Lynxtrap]

Some corrections in the schematic, including the Send and Return jacks for the D-lator.
They seem to be connected in such a way that signal is passed with nothing plugged in. The only reason I can see for that is to use the loop as some kind of boost.
Outboard 'lators are used like that as well, so I guess it makes sense?

Mad Cat Special.png

FX Loop connection.png

Mad Cat Special schematic_rev2.pdf

[Lynxtrap]

I have now powered this thing up, and there is sound. Kind of, with all sorts of funny stuff going on.

The major cause and problem at this point is that the relays get about 1.6V. They need 5. I get a reading of about 2.5V at the output of the small rectifier that is fed by the filament wiring, 6.4VAC in this case. That is even after disconnecting the relays from their power supply.

I’m not sure where to start looking for faults here. The rectifier? Something else stealing the power?

Relays, transistors and such is pretty much a black hole for me. I don’t even understand how the relay circuit works,in detail, to be honest.

I had really hoped that there would not be any issues with this part of the amp. I can handle the rest.

[Lynxtrap]

Does this look like a working switching circuit? There is no connection to ground.

6D97B12E-B0C9-4C66-B0DE-C81E7FB915A1.png

[pompeiisneaks]

relay power supplies should be 'floating' so no ground to chassis.

A quick glance seems to show that as right, but better eyes than mine should be consulted

~Phil

[alkuz1961]

The major cause and problem at this point is that the relays get about 1.6V. They need 5. I get a reading of about 2.5V at the output of the small rectifier that is fed by the filament wiring, 6.4VAC in this case. That is even after disconnecting the relays from their power supply.

Probably the heater winding has a central point and therefore the relay circuit receives only half the voltage.
Also, with such a small excess voltage, it is important to use LDO chips for Q1, like LP2950, L4940V5 or similar. Typical 78L05 will not work normally.

[Lynxtrap]

Probably the heater winding has a central point and therefore the relay circuit receives only half the voltage.
Also, with such a small excess voltage, it is important to use LDO chips for Q1, like LP2950, L4940V5 or similar. Typical 78L05 will not work normally.

Thanks! Yes the heater winding has a center tap. Any suggestions how to get the needed 5V to those relays, other than the transistor?

As mentioned, I have never worked with relays before and basically know nothing. Are they supposed to have minus voltage on one side (which seems to me like the result of this setup)? Like +5 on one side and -5 on the other?

[talbany]

Ideally your power supply for your relays should?
1. Have it's own separate ground reference
2.Be regulated
3.Well filtered
The filament supply in a tube amp should be balanced and isolated or will be susceptible to all kinds of noise hum issues (especially a high gain amp). Some here have experimented with a floating supply taken from the filaments and have had some luck. If you don't know how to do that my advice would be go on E-bay and find a 5/12/16V transformer and stick it in the amp they are cheap! and build a seperate supply thats filtered regulated that way you know your isolated and clear from any possible noise problems injected in your filaments that can drive you absolutely nuts!!

Study this discussion
https://ampgarage.com/forum/viewtopic.p ... s&start=15

Tony

[Lynxtrap]

Thanks!
Perhaps something like this would work? https://www.tube-town.net/ttstore/print ... 0v-6v.html
One problem in this case is real estate. The relay supply card sits over by V1, and that's basically the only space available in the chassis. I don't like the idea of 230VAC leads across the whole chassis and a transformer + rectifier at the input

By the way, here are a couple of pics of what I have to work with... The open space by the filter caps is occupied by the OT underneath.

01.jpg

03.jpg