JTM45 / 50 Build

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Spencerleehorton

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Fixed up the router table, made a few adjustments and set to 12mm finger joints.
Glued and will wait until dry then finish off the inside supports and front and back panels.
Will put in a brass mesh on back panel.
Will also need to route out the beading lines.
Then will cover with tolex and beading.
 

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john12ax7

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This is true diy,  electronics,  metal work,  and wood work too.

Regarding capacitors, you're probably already past this part,
but film is better in the signal path if possible.  Though you might need to go electrolytic if the cathode bypass cap is really large.
 

john12ax7

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If you really want to go down the rabbit hole you can try different types of film, Mallory 150, Orange Drops,  Mojotone Dijon,  Sozo mustard caps, are some common guitar amp choices.
 

Spencerleehorton

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i was mostly using polyproperlene for the caps and mica for the smaller ones, these seem to fit into what people prefer tonally. where i was going to experiment was the cathode bypass caps, as ushally used are electrolytic but i have read that film in these places is tonally better?
 

Heikki

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Spencerleehorton said:
i was mostly using polyproperlene for the caps and mica for the smaller ones, these seem to fit into what people prefer tonally. where i was going to experiment was the cathode bypass caps, as ushally used are electrolytic but i have read that film in these places is tonally better?

Any nonlinearities any type of cap might have won't matter in a guitar amp. Distortion from tubes will always be much higher than any distortion caused by capacitors. If you plan to sell the amp then use whatever snake oil caps gets you the best price.
 

Spencerleehorton

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Testing this power txf I have which has a centre tapped 190vac, so its 2 x 190vac or 370v ac it tests.
My only worry here is that after the diode bridge I'll be up in the 500v dc area rather than 440v dc!!!
The other windings are 60v, 20v and 2 x 6.3v.
I did want to try this txf but now I'm thinking it'll be a problem?

Any thoughts on what I can do?
 

john12ax7

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For a plexi type 0.68u cathode bypass cap go with film.  But the jtm45 schematic I've seen used 330u. You will have a hard time finding film in that size.  It will also be very physically large and expensive. So you probably need to go electrolytic in that spot.
 

Spencerleehorton

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yep spot on John, any ideas on what i should do with getting my B+ to 440v dc?

With my power transformer i can get 370v AC but after diode bridge that would give me 518v!!

Whats a good way to comfortably reduce it 50-60v?
 

john12ax7

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You could string together some high power zener diodes, but  that would still be a decent amount of heat and wasted power to dissipate.  I would suggest just getting a different more appropriate transformer if that's possible.
 

Spencerleehorton

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Yeah ok, dumping 80v ac is a bit difficult and tbh I'm not even sure what rating these windings are so could lead to having some hum issues, probably best to just get the proper power txf as you say.
 

Spencerleehorton

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this is the third time ive tried to put this post up!!

ive made my own dual cap in the V2 pin 6 position, its only 350v is this going to enough or should i have at least 450v here?
 

john12ax7

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Could you post a schematic? Would make it easier to identify things like V2.

For an output tube plate a 350V cap will almost certainly be too  low.  Possibly acceptable for a preamp tube.

Guitar amps will often use a half wave rectifier with a center tapped power transformer.  You might see something like 700VCT or 350-0-350 speced for a Marshall. In that case peak voltage after rectifier could be viewed as 0.7x of the 700 or 1.4x of the 350, either way around 490V. In practice the loaded transformer will be less than this,  around 1.25 or 1.3x, so you might get 450V with the aforementioned transformer. That's with diodes,  a tube rectifier will drop it a bit more. Note a tube rectifier is half wave,  as are two diodes. A four diode bridge is not as common in guitar tube amps.

So you definitely need high rated caps at the early positions.  Marshall will often use 500V can caps. Some builders will use two 300V caps in series with resistor dividers across them.
 

Spencerleehorton

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Yes sorry here is the schematic I'm trying to work off.
Could I put two 350v caps with resistors and resistor dividers after the diode bridge to handle the 520v dc

On this schematic its half wave, but I will be doing full wave bridge rectifier.
Is this the simplest solution?
 

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Spencerleehorton

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So hopefully my best bet is to have 2 x 47uf 450v with 2 x resistor dividers after the diode bridge and change that other dual cap for 2 x 33uf 450v?
Then I should be good right?
 

Spencerleehorton

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My other option I thought of was to take out the power txf and output txf from this egnator tweaker 40 amp and see what they are like, then if they are ok use them and if it has a choke in there use that and use the normal config?
 
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