Heathkit A7E Conversion to Guitar - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 17th February 2007, 01:46 PM   #1
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
Default Heathkit A7E Conversion to Guitar

I received the HealthKit A7E amp yesterday and it sounds pretty good for guitar, although there are some issues that need to be addressed: It sounds a bit muffled, and there's significant noise. On the good side, it's only 6 Watts and breaks up nicely at low volumes, which is why I purchased it.

I'm considering the following mods and upgrades:

1. Replace mono input 1 with a guitar jack
2. Replace the power cord with a 3 prong and ground the chassis
3. Replace the cathode 10F/25V cap on the 12SQ7
4. Replace the cathode 20F/25V cap on the 12A6's
5. Bypass the RC filter circuit after the first gain stage
6. Review and revise the star grounding

The 20F/25V cap is within the capacitor can with the power supply filter caps.
The entire can might need replacing since it's probably old, but I'll see how it sounds with the initial mods.

The grounds are all connected to the input jack, which you can see in the wiring photo below (I'll take a better picture and post it).

Here's the schematic
Click the image to open in full size.

And the stock wiring
Click the image to open in full size.

Does this sound like a good plan?

Do you have any other recommendations or suggestions?

  Reply With Quote
Old 17th February 2007, 02:02 PM   #2
tubee is offline tubee  Netherlands
diyAudio Member
tubee's Avatar
Join Date: Jul 2004
Location: Netherlands

Use relative small coupling caps to get a bright sound for guitar instead of muffled. But they are relative small allready i see.
Daisy Bell
  Reply With Quote
Old 17th February 2007, 02:27 PM   #3
diyAudio Member
Tom Bavis's Avatar
Join Date: Jun 2002
Location: Macedon NY
Those yellow Astron caps are leaky - trust me. Measure grid voltages (should all be zero) and you'll see.
  Reply With Quote
Old 17th February 2007, 02:55 PM   #4
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
I've made a few changes and tested, and it now sounds fantastic! Changing to a three prong ground cleared up all the grounding issues.

I also changed the input resistance to ground to 100K (the schematic says 27K, but there were two 100Ks in parallel). Should I change that to 1M?

I do need to pin the treble all the way up in order to get a reasonable tone.

What do you recommend for removing the filter network between the first two gain stages? Is this some kind of mid boost?

Should I take everything out, except for the .05F coupling cap?
Or, should I leave in the 470K to increase the input resistance?

Perhaps the problem is the values in the Bandaxal tone circuit, which I assume are meant for stereo instead of guitar.

  Reply With Quote
Old 17th February 2007, 03:37 PM   #5
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
I checked voltages, and all the grids are zero, except for the 12SN7, which shows 4mV.

The power supply runs a little hotter than the specs on the schematic, except for C+, which is lower (specs are in parenthesis):

A+= 403 (390)
B+ = 382 (360)
C+= 145 (180)

The cathode bias (12A6 pin 8) is at 26V
  Reply With Quote
Old 17th February 2007, 05:48 PM   #6
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006

OK, I took out the tone circuit and the volume dropped to barely hearable, put it back in and the same. Then I noticed I had swapped the 12SN7 and 12SL7 tubes. I traded them, put the circuit back to the way it was before, but still the same.

I checked the power supply voltages and they're the same.

I wonder if I burned one of those input tubes, or if one of those Astron caps gave out. The hard part is these are unusual tubes, so it's difficult to do a swap.
  Reply With Quote
Old 17th February 2007, 07:07 PM   #7
diyAudio Member
Dave Cigna's Avatar
Join Date: Oct 2003
Location: Finger Lakes, NY
The filter network between the first two stages looks like a RIAA equalization network for phonos. It provides a serious bass boost (or treble cut) which explains your need to dime the treble control. You can remove it by clipping out the .0035 and .01 caps as well as the 22k resistor. You can leave the 470k resistor there. It cuts the gain a little, but it looks like you have plenty of gain already. You can remove it later if you decide to.

Hope you figure out what's gone wrong...

-- Dave
  Reply With Quote
Old 17th February 2007, 08:21 PM   #8
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
Thanks Dave, that did seem to work.

It does seem as though one of the gain stages isn't working, which is most likely a bad tube, since all the voltages are fine.

On the bright side, it did sound very nice for a little while
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Guitar amp conversion DigitalJunkie Instruments and Amps 1 7th November 2008 03:16 AM
Heathkit EA-3 conversion to guitar amp Jozer99 Instruments and Amps 9 10th August 2007 06:12 AM
Heathkit A7E for guitar - preamp gain PRNDL Instruments and Amps 6 16th March 2007 06:17 PM
Scrap Heathkit W-5m for guitar amp? Tim Wyatt Instruments and Amps 3 23rd February 2005 08:00 AM
converting Heathkit AA-191 to a guitar amp Rich Wolski Instruments and Amps 1 5th December 2004 06:03 AM

New To Site? Need Help?

All times are GMT. The time now is 01:39 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2