My3gger

Reusing chasis for 18W or JTM45 like amp
« on: April 21, 2018, 02:09:05 PM »
Hii,
i bought this FBT R2 amp with EL503, with low Raa OT at ~50W for sweep tubes. It is built with two metal chasis (preamp/psu-amp) with spring reverb into sturdy wooden head.

https://www.mercatinomusicale.com/mm/a_fbt-500-r-2-cassa_id5164046.html

Most times i don't need more than 18W,  "mini" Marshall Bluesbreaker like head should sound good, don't annoy neighbours and preserving my hearing. Low power goes to monitors when playing with drummers, most of it is acoustic beside one guitar.
Seems pitty not making 35W version of Bluesbreaker because more people could use it and have reference sound, it isn't  comon here in ex Jugoslavia...
Head already has controls/inputs at positions for bigger version and can disipate 50W easily. I never use more than ~30W, so parts aren't much more expensive. EF86 channel would be very easy with so low electro/mechanical coupling.

http://4tubes.com/SCHEMATICS/Music-amps/Marshall/18wr.gif
http://hamstercagethemovie.com/amazing-car-subwoofer-amplifier-circuit-diagram-photo-inspirations/vintage-schematics-marshall-wiring-diagram-components-2/

18W or 35W versions are quite rare, not many here know both by heart. Is attenuator between output and speaker any good for getting saturated 30W amp at lower levels? Many from Gdiy built 18W variants, does it sound better or is it just matter of power and small differences in preference?
I'm also open to other suggestions if it is blues rock oriented saturated sound.


walter

Re: Reusing chasis for 18W or JTM45 like amp
« Reply #1 on: April 30, 2018, 01:55:33 PM »
How does it sound as is?  The circuit that I found on-line looks pretty typical fender type. I notice a few 100k capacitors,google tells me  .001uF . There are no reference designators on the schematic that I found. Maybe try to replace some of those caps. After the third tube half on the Bass input, replace the 100k cap with .1uF. The next tube half before the phase invertor, replace the 100k cap with .047uF. The Two 100k caps after the phase invertor replace with .02uF.  I don't know what the amp looks like or how it is to replace components. Some amps are good for gutting and starting new, others may just need a minor tweak.
Blown like a fuse

abbey road d enfer

Re: Reusing chasis for 18W or JTM45 like amp
« Reply #2 on: May 01, 2018, 02:32:47 PM »
I notice a few 100k capacitors,google tells me  .001uF . 
Actually 100nF, or 0.1uF.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: Reusing chasis for 18W or JTM45 like amp
« Reply #3 on: May 01, 2018, 02:49:28 PM »

I'm also open to other suggestions if it is blues rock oriented saturated sound.
Unless you gut it entirely and keep only the chassis and cabinet, if you want to re-use at least the power transformer, you could not make an 18W amp, because the B+ voltage of 370 V suggests about 30-35W.
I've found that Italian amplifiers of the era did not sound very good, although everything in them looked apt at sounding good. I would think it boils down to output transformers and loudspeakers.
I would think you could build a nice Super Reverb AB763; it would be slightly less powerful because of lower B+ (370 vs. 470V).
You would need a new output transformer and a set of 6L6's.
You may want to evaluate the actual speakers with another head and decide if you want to keep them or install a newer speaker.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

nashkato

Re: Reusing chasis for 18W or JTM45 like amp
« Reply #4 on: May 03, 2018, 11:59:03 AM »
Hi
I was looking for quite some time for a pair of EL503´s
They´re hard to find and expensive .
I eventually gave up and sold the amp as it was .
EL503´s are very nice power tubes , they´re the successor of EL34 introduced in 1966 .
an AB amp can put out 40 Watts
So what you go there is more in the league of 80 Watts probably
please don´t gut it !
(maybe we can arrange some kind of trade for something from the low power region ?)

btw : i guess you must be from Slovenia ? that is the only ex-Jugoslavian state , right ? i´m about 1,5 h away from Ljubljana
« Last Edit: May 03, 2018, 12:02:15 PM by nashkato »


 

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