wiki:GretchensAmp

Gretchen's Amp

What I want here is a simple, trouble-free build with modest power for my sister who lives in a retirement community. Going for an easy build, good sound and readily available tubes. 3-4 watts should be plenty. I like single-ended ultralinear designs for their good performance, classic tube sound and low parts count.

This seems like a winner: https://www.cascadetubes.com/the-6v6-marblewood-amp/

It has tube rectification (which I like) with one choke then simple R-C filtering. His wrap-around wood chassis with metal top plate is also a construction style I favor.

2  GXSE10-8-5K          $37.99    $75.98    Edcor
1  XPWR106              $52.74    $52.74    Eecor
1  158Q 5H choke        $22.16    $22.16    Mouser
2  6W6 tube             $ 5.00    $10.00    eBay price
1  12AU7 tube           $10.00    $10.00    eBay price

2022-05-10

Left channel dropped out. Took off the bottom and checked voltages. Problem went away, sigh. For the record:

http://ohm.bu.edu/~hazen/TubeAmp/GretchensAmp/schem_voltages.jpg

Preparing for delivery. Attached bottom screen and feet. Looking for spare tubes. Operating preamp tube is ECC83S (12AX7). Tried an ECC82 (12AU7) and the gain is noticeably lower.

Need to buy:

  • (1) JJ ECC83S
  • (2) JJ 6V6S

2020-06-01

Blew a 1A SLO-BLO fuse yesterday. Replaced it, but maybe should be using 2A?

Sometimes there is strange digital-sounding noise. Not clear if it is coming from the phone. Need to investigate.

2020-05-04

Replaced JJ ECC82 (12AU7) with JJ ECC83 (12AX7). Significantly more gain. I like the sound better. I think we have a winner!

2020-05-03

Replace the old RCA 6D6DG with new JJ 6V6S. JJ ECC82 (12AU7) preamp. Bias point is much more comfortable, 41mA vs 56mA. B+ is now close to 300V where it should be instead of 243V. Sound is excellent! Plenty of output with iPhone using Minimus-7s. Have ordered a couple of other preamp tubes (12AX7, ECC802) to try.

http://ohm.bu.edu/~hazen/TubeAmp/GretchensAmp/pix/complete_JJ_sm.jpg

2020-05-01

  • Tube Depot: 9400136895232575091922
  • AES: 9400115901405920472188

2020-04-26

Chassis built, all mechanical parts installed. Wired the power supply and tested with 3750 ohms (2x 30W 7.5k parallel). Measure voltages:

  • AC line: 117VAC
  • Cathode of rectifier: 328V
  • After choke (on 100uF): 292V

Seems OK for now, possibly want to tweak up later.

2020-04-21

All parts in hand! Removed the groove in the chassis to the top plate drops in, and glued it up with corner blocks. Ready to go now. Only missing one rotary switch for input select. Could steal the non-functional one from the computer amp.

2020-04-17

Most parts in hand except Edcor transformers which are due Sunday! Need a few more items:

  • Input selector, AES P-H395 rotary switch and P-K349 knob
  • More input jacks (S-H267R and S-H267W)
  • Bigger grommets (EDF?)
  • Copper clad for inputs (EDF)
  • Coax cable for input wiring (EDF)

Probably hold off on this stuff until most of the chassis is done. Unlike previous builds, I'd like to do *all* the drilling and a trial mounting, then disassemble everything and prime and paint the top plate flat black. I've made life a bit hard for myself with the top plate integral to the wood chassis too, so I can't glue up the chassis until after the top plate is completely finished. (next time, do it differently!)

Early photos:

http://edf.bu.edu/svn/edf/EricHome/TubeAmp/GretchensAmp/pix/top_layout_sm.jpg http://edf.bu.edu/svn/edf/EricHome/TubeAmp/GretchensAmp/pix/bare_chassis_sm.jpg

Last modified 3 months ago Last modified on May 11, 2022, 8:39:48 AM