Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 04, 2023, 07:56:59 am
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I'm wondering how the switches work in the Robben Ford's Dumble ODS.
There is in the front panel Rock/Jazz switch that is named as PAB (Pre amp boost) in the schematic.
It is controlled by relay.
Overdrive setting is also controlled by relay.
https://i.imgur.com/8Bq0qf5.jpg (https://i.imgur.com/8Bq0qf5.jpg)
Then there are in the back panel manual switches both for OD and PAB (Rock/Jazz) in order to activate them if foot switch is not used.
https://i.imgur.com/me62Igp.jpg (https://i.imgur.com/me62Igp.jpg)
What is the idea of the Rock/Jazz switch in the front panel and is it parallel with the PAB switch in the back panel i.e. doing the same thing?
Or are there some other functions that are not visible in the schematic.
/Leevi
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I'm not sure I am understanding the question The PAB "sort of removes" some of the tone stack giving a pre amp boost. The treble and bass pots are rendered ineffective when the PAB is on.
The jazz/rock switch is something different than the PAB and creates a softer/jazz tone or more of a rock tone. There is alot of information on this on The Amp Garage. There are some YouTubes that show the difference in tone between the jazz/rock setting.
Dumble ODS - Jazz/Rock Switch - YouTube (https://www.youtube.com/watch?v=kwC0Jlcz7Tc)
If I have not understood the question, I apologize.
with respect, Tubenit
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Thank you Tubenit, you have correctly understood the question.
There are two relays in the circuit and one of them ("RLY1 PAB") is marked to control the Rock/Jazz function.
The other one controls the OD.
Of course there can be faults in the schematic.
I have to check the Amp Garage.
/Leevi
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There are two relays in the circuit and one of them ("RLY1 PAB") is marked to control the Rock/Jazz function.
After another check I noticed that I have misunderstood the control of the Rock/Jazz switch.
It is not controlled by the relay! It's an own mechanical switch. The jazz/rock switch is something different than the PAB like Tubenit mentioned.
/Leevi
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There are two relays in the circuit and one of them ("RLY1 PAB") is marked to control the Rock/Jazz function. The other one controls the OD.
I'm reading the schematic differently -- maybe not correctly.
My understanding is that the PAB and OD circuits can be engaged by either the back panel switches or by the foot switch -- this is definitely how my clone operates.
The Rock/Jazz switch is located only on the front panel. When I operate the PAB and the Rock/Jazz switches, I get 4 distinct tones.
It appears to me that the Rock/Jazz switch does interact with the PAB relay, but is independent of it.
For sure it is a complicated pre-amp! But it does sound good, so good.
Edit: OK, I see we're on the same page. One of my "planned" projects is to go through that schematic and trace the signal path for each setting, so as to understand it better.
My clone was built with the AAelectonicspcb. His schematic is higher resolution, and I had it printed into a 17x22 sheet -- that really helped my old eyes.
I really like this amp.
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Thank you for the info. I'm going to build a Dumble clone in the near future. Do you think if the NTE452 jfet cam be replaced by some other type of jfet like LND150?
/Leevi
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As I read the drawing:
Relays have coils.
Switches don't.
These relays are drawn with pointy contacts.
These switches are drawn with round contacts.
Round vs pointy is meaningless on the surface; just a matter of style. But it looks like the drawer took clipart from two different sources. Consistently.
I also note that the relay diodes are probably backward.
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...Do you think if the NTE452 jfet cam be replaced by some other type of jfet like LND150?
That's not even the same kind of an FET. And the original may be quite critical for input overload and bias.
Yes it will cost you $10++ shipped but you can still buy the NTE452 part.
https://www.ntepartsdirect.com/ENG/PRODUCT/NTE452
https://www.tme.com/us/en-us/details/nte452/tht-n-channel-transistors/nte-electronics/
https://www.amazon.com/NTE-Electronics-NTE452-N-Channel-Transistor/dp/B00DJYB9N6
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I also note that the relay diodes are probably backward.
I thought so too, but they work as drawn.
I simplified the foot switch drawings a bit, but it makes sense.
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Yes it will cost you $10++ shipped but you can still buy the NTE452 part
This is a small cost compared to wasting time with some replacement fet.
/Leevi
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An additional question regarding the Dumble ODS.
What is a recommend setting of the 10K trimmer after the NTE452 FET?
The amplification is very high even with low setting since the FET is cascade connected with the V1.
/Leevi
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Lots of info about that over at the friendly TAG forum.
https://ampgarage.com/forum/
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Are most aluminum covered E-caps we buy have the aluminum cover connected to the ground lead? No issue when using in parallel but series would be a different case...?
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Are most aluminum covered E-caps we buy have the aluminum cover connected to the ground lead? No issue when using in parallel but series would be a different case...?
The aluminum shell is connected to the ***NEGATIVE*** lead. The negative lead is not always connected to ground. Bias caps are one example where the negative lead is not connected to ground. Series stacked caps in the power supply is another example where the negative lead of the top cap is connected to the positive lead of the bottom cap.
What issue do you have when connected in series? And why is this in a Dumble ODS thread?
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Sorry l posted this in this thread, l hit a link posted in this thread that lead me to this E-cap question, my mistake.
My saying ground should have been saying negative lead, my mistake again but l did not know this. So if one touched a bias e-cap’s case, l assume there is negative voltage on the case, since negative voltage, no shock..?
Series caps would have positive dc on the case, that’s dangerous. I never see schematics or amps that run series E-caps cover the case, so touching two 350v caps outer cover when in series is a danger.
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All the axial and radial e-caps I have seen have a plastic insulating cover over the aluminum shell. Should be minimal danger. And even multi-cap cans have phenolic mounting plates and cardboard tubes available to cover the can in situations where the negative shell will not be connected to chassis.
BTW, -100VDC will bite you just as hard as +100VDC.
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BTW, -100VDC will bite you just as hard as +100VDC.
I didn’t know that, thanks. Yeah, there is a coating around the aluminum. Hey, Steve sometimes l’m an ass but don’t mean to be. :BangHead:
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I've never thought that.