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Old 22nd July 2014, 04:46 AM   #1
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Default What's wrong with this chassis layout?

What's wrong with this chassis layout?
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Old 22nd July 2014, 07:13 AM   #2
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A Tube rectified guitar amp, about 50 Watts?

The layout is good for distribution of weight from the trannies but you have a couple of potential problems. The high voltage (DC and signal level) feeds to the output tranny cross the entire width of the chassis. That may cause instabilty from inadvertent feedback.
I'd be tempted to forgo the mechanical balance in favour of shifting the output tranny closer to the output tubes.
The rectifier and output tubes are the major source of heat, I would spread them out more.
Ditto the small tubes, in that case however it is for stability rather than heat concerns.
left most small tube the power amp phase splitter, going to teh right have the reverb driver/recovery next then the 3 preamp tubes with eqaul spacing across teh remaining width. In high gain amps it is good to keep the preamp tubes physically separated.
My 2c worth.
Cheers,
Ian
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Old 22nd July 2014, 03:09 PM   #3
Tesla88 is offline Tesla88  Italy
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A schematic or an idea of what amp are trying to replicate would be nice, and starting from a already existing could help (a JTM45 + reverb ?)
Anyway, i would move rectifier and power tubes back with the rectifier near PT and finals near OT . Preamp tubes on front, you need more space between those ...

Is it easier if layout of preamp tubes follow the signal path ..
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Old 22nd July 2014, 03:56 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Here are a few thoughts,

You need to keep any tube at least one to one and a half tubes width from any others and if the tubes are different small and larger side by side its the width of the largest tube you use to measure.
If you have tubes close by other parts its one and a half to two widths away.

If they are by capacitors its two widths away.

If you want to put the transformers as they are I would put the power at one end with tube rectifier and the mains supply and switch..
The output transformer and output tubes at the other end ..
And bring the input and preamp tubes in the middle with as much space between each end as possible. keep the drivers by the output tubes an preamp in the middle.


In the end its up to you.
You could even put the rectifier and power tubes between the transformers on one side and the preamp tubes on the other and still keep the rectifier by the mains Tx and the output tubes by the op Tx.
Then you could input to the preamp tubes and keep all mains wiring on the one end of the chassis, the output on the other end and have short wiring for the signal cables in the middle. Distances between are critical.

Here is a quick scribble..you could try the layout and see what you think..play about with the position of the rectifier and the chokes. Imagine the chassis in three parts mains/ pre/ and output.

Good luck with your build..

Regards
M. Gregg
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Old 22nd July 2014, 04:23 PM   #5
kevinkr is offline kevinkr  United States
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Will move this to the instruments & amplifiers thread - you'll get a lot more useful input there I should think from fellow guitar amp builders.
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Old 23rd July 2014, 05:54 AM   #6
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The controls are mounted on the back panel, or the valves are mounted on the front panel?
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Old 24th July 2014, 12:47 AM   #7
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Keep mains transformer away from tubes and audio signals.
Keep mains wires well away from audio wires.
Output transformer near to output valves.
Leave plenty of space between tubes to get rid of heat.
Keep audio signal wires as short as possible and as close to tube they feed as possible

A circuit would be good to criticise.

I have just been through a major overhaul of a valve mixer and learned quite a bit about minimising hum. Raise heater to DC offset of 45 volts.
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Old 26th July 2014, 03:29 AM   #8
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Thanks guys for your help! This is what I'm up against; I'm definitely gonna have to keep the transformers in that configuration because of no room around the speaker. Another question; do I really need a choke coil in ultra linear operation?
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Last edited by robbieh79; 26th July 2014 at 03:57 AM.
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Old 26th July 2014, 07:40 AM   #9
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by robbieh79 View Post
Thanks guys for your help! This is what I'm up against; I'm definitely gonna have to keep the transformers in that configuration because of no room around the speaker. Another question; do I really need a choke coil in ultra linear operation?
Why cant you put the power supply (Transformer rectifier and capacitors /choke) on a sub chassis in the bottom of the amp and put a plug and socket with retaining clip or screw on plug from the top amp chassis to the bottom sub chassis. or is there a reverb in the bottom?

Put the output tubes by the output Tx and the preamp tubes in the middle with the driver by the power tubes and the preamp tubes as crescent in the
middle. Then you can still have the power switches etc as they are now in the top section.

If you did that you might even be able to mount the output transformer above the power switches and put the preamp section where the output transformer is now. That would mean the preamp section is away from all interference.

That is what I would do if you don't have a reverb. have a sub chassis..Gretsch did it in there amps similar to this..


https://www.google.co.uk/search?q=gr...ml%3B640%3B480

Here is another one with the preamp and power amp separate..power from the power amp and PSU feed the preamp section.


https://www.google.co.uk/search?q=gr...2F%3B800%3B600

And no you don't have to have a choke in PP..you can just use capacitors and resistors..but you need to be careful you don't get sag so be careful with the values.

Regards
M. Gregg
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Old 26th July 2014, 08:13 AM   #10
M Gregg is offline M Gregg  United Kingdom
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NB..if you don't use a choke then I would forget the tube rectifier and go SS diodes..the problem with not using a choke is limiting the charge pulses (current). The first capacitor maximum value after the rectifier is critical..so if you don't use a choke go for SS diode rectifiers.

Value of the first capacitor is critical after a tube rectifier..if its to high a value it will burn out the rectifier and you will have reliability issues..(fail on stage)

Regards
M. Gregg
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