Dumble 70's problems Need help

[Dino]

Hi i'm fairly new to amp garage and i built a 70's dumble from diagrams i got of the internet but i cant seem to get it to work i've followed the diagrams and checked it over a few time but always have the same problems am i missing something or are the diagrams wrong::
the problem is strange squiling noises when standby is on
Here are the diagrams if you can help aor have an idea let me know.
Dino

[Sonny ReVerb]

Did you try reversing the leads from the power tubes to the output transformer? You can test to see if this is the problem before switching the leads by temporarily disconnecting the NFB (the 3k9 resistor between the output jack and the phase inverter). If this solves the problem, you need to switch the leads because you are getting positive feedback instead of negative feedback. It's a fairly common issue on new builds (Murphy's Law).

Make sure you take all the proper precautions of draining your filter caps, etc. before you do any work under the chassis. There are lethal voltages present.

[Dino]

Yes i tried swapping the leads and other ideas but still the same.
Thanks

[Structo]

Actually the RF mod replacing the choke should be 300R-500R ohms.

The choke does have a DC resistance but with the added inductance it acts like a much larger opposition to the change in current so that is why a larger resistor is used.

What is your voltage at point A on the power supply?

[Dino]

The voltage is 1 or 2 Volts off about 458V instead of 460V all of the voltages are very close to what they should be but i'll check them monday at let you know the exact voltages

[Structo]

Don't sweat small differences.

That is fine for B+.

You will just have to adjust for the preamp voltages.

[crbowman]

Hi! Welcome Dino.
At least part of the problem is with the schems you are working from.
I've seen these as well and IMHO they are not really accurate. They were a good-faith effort and a best guess for their time, but there is much more accurate info available here. I've built a 70's style as well but based it primarily on this schematic:

https://tubeamparchive.com/files/40_schematic_496.pdf

with the additional corrections Marcos has supplied in this post:

https://tubeamparchive.com/viewtopic.ph ... highlight=

Here is a great layout Tony has done for a 70's style amp:

https://tubeamparchive.com/download/file.php?id=10532

I would at least compare your work to this info. Much more accurate.
Good luck! Feel free to PM me if I can be of further assistance.

[Dino]

Thanks for all the help the new diagrams are completeley diffrent from the one i used will try these and let you all know.......

THANKS THANKS THANKS

[Bob-I]

Another thing, post pics of your build, there might be some layout problems that can be seen.

[talbany]

Dino
If the amp is making noises in STBY then the issue is most likley in the output section.. Make sure you have the output tube sockets wired correctly..(Check Screens for voltage) you should have maybe 40v dc on screens in STBY..Also check to make sure your OPT is wired properly..You could also have the STBY wired wrong.. It's related to power tubes and output transformer STBY wiring..Hope this helps..My Guess

FWIW.. Reversed NFB won't squeal when the STBY is on

Tony

[Dino]

Hi guys just a quick update on my Dumble 70's build fixed the problem with the squilling it was the ratio pot on the overdrive channel i had wired the pots backwards so they were full on and not off as i thought so the amp was feeding back in clean....

I've now got the amp running and sounding half descent but have a new problem i cant get the bias down to below 110mv so after 1-2 minuets the valves start to glow red so i checked all the voltages and there all LOW even with the 80ohm resistor think the problem might be with the PT or OT which are in the jtm44 case i'm using for my prototype..

But am making progress one step at a time (With help from YOU GUYS)

THANKS THANKS THANKS for all your continued help and support

[Structo]

Change the 27K resistor on the bias pot.

Or maybe temporarily hook up a pot in place of that resistor.

Pull the power tubes and experiment with the bias voltage.
You want more negative range in voltage.
Measure this on pin 5 of the power tubes.

In other words, say you have -40v, you want it more negative like -50v.

[talbany]

Hi guys just a quick update on my Dumble 70's build fixed the problem with the squilling it was the ratio pot on the overdrive channel i had wired the pots backwards so they were full on and not off as i thought so the amp was feeding back in clean.... :D :oops: :lol:

I've now got the amp running and sounding half descent but have a new problem i cant get the bias down to below 110mv so after 1-2 minuets the valves start to glow red :?: so i checked all the voltages and there all LOW even with the 80ohm resistor think the problem might be with the PT or OT which are in the jtm44 case i'm using for my prototype..

But am making progress one step at a time (With help from YOU GUYS)

THANKS THANKS THANKS for all your continued help and support

Dino
You may want to think about rewiring the switching relay like 124.. this way it grounds out the OD side when in clean..This will help with bleed back ghost notes at high volumes and most important noise generated from the OD side... Take a look at the bottom of this thread as Marcos rewired his 70's and found a noticeable improvement..
Your bias range should sweep between -35 to -65 V or there about's.. Check it before you pop in the tubes and risk frying them.. good luck!!..

Tony

[Dino]

Hi checked the voltage on pin 5 and found it to be -2.3VDC so i removed the 27K resistor and now its -41.4VDC so im going to fit a 10K pot so i can fine adjust it because the bias has now dropped to 2MVDC and ideally i want it around 37MVDC so just some fine tuneing and my prototype will be ready for a burn test..........

Just to let you know this proto type has no relay's just 3 flickswitchs that will all come in the first model and i will post some pics when i get that done.

still got some testing to do will keep you all aprised
Thanks

[Dino]

The problem is the transformer i'm using it dosent have a 60v tap so i took a feed from the mains and droped it down to 60v buut that ruined the waveform so i've shelved the project till i get chassis,it & ot. I have already laid boards out ready.
Thanks Dino