Instrument amp from '59 Lowrey donor chassis. - diyAudio
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Old 23rd December 2011, 01:57 PM   #1
diyAudio Member
Join Date: Apr 2010
Default Instrument amp from '59 Lowrey donor chassis.

Hi All, I recently got a bee in my bonnet to build an amp after I found a '59 Lowrey organ chassis (complete) on ebay (haven't played with tubes in many years). Long story short, I have it built, based on a mix of a 5F6A (output) and Marshall Bluesbreaker (preamp) designs.

Problem is that the power transformer gets a lot hotter than I would have expected. Rectification is SS fullwave, yielding a +B of about 520V. To keep the temp buildup in the PTX so it can still be touched I run it with very cold biasing (10mA), for a total HT load of under 30mA at low-medium listening levels. By my calculations the HT winding should be able to deliver 80mA without blistering the paint. I am using a pair of used Magnavox 7027A tubes (like them more than the cheap 6L6GCs I picked up)

PTX measured/calculated specs:
120V PRI (original fusing was 2A Slow Blow, estimated 175VA)
HT - 475-0-475VAC (also fused at 2A SB)
Fil 1 - 6.5CT
Fil 2 - 6.5

Original Tube Complement:
2 x 6AU4
2 x 6L6GC
2 x 6SN7GT
1 x 6SL7GT
2 x 6X8
5 x 12AX7 on a separate chassis

Calculated loads:
HT - about 150mA minimum
Fil - 9.3A

Other Transformers on the chassis:
Filament (about a 175VA core, implying about 27A, most probably for
the other 57 6X8s @ .45A each)
Output - Approx 40W

So, my question is: What could be the reason for the unexpected thermal reaction of the PTX, and what could one do about it (short of rewinding it)?
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