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Dirty-South Blues Harp forum: wail on! >
Video - Amp Mods/Experiments for Harp
Video - Amp Mods/Experiments for Harp
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rogonzab
580 posts
Aug 12, 2014
6:11 PM
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Video description:
21/07/2014
This video provides a brief audio sampling of my harp amp experiments (modding and building) since taking up the harmonica just over two years ago. These experiments have been eye opening for me, helping me better understand my own amplified (and even acoustic) harp tone, as well as the workings of the gear itself. I hope elements of this video prove useful to others seeking their own understanding of this part of their tone.
As for the audio samples; please forgive the noodling. For some measure of consistency I tried to stick to the same kind of vamp material each time, as well as a tight-cupped tone. These vamps are completely tongue blocked; thanks to Dennis Gruenling for helping me expand that part of my blues technique.
Bugera V5
Two months after taking up the harmonica, the audio engineer in me became curious about the mics and amps used to sculpt great blues tone, as well as what modifications I could make to amps to make them better for harp. I started with a Bugera V5 tube amp due to the low cost and slightly expanded feature set it offers (power soak, headphone out, reverb, and both gain and volume controls).
First, I ordered a new cabinet from Weber. As part of the new look, and to accommodate the dimensions of the new cab, I cut up the chassis and reworked it into three parts (Main, Power and Output). My electrical mods were fairly straightforward, and outlined in the video. Replacing the original 20k linear taper reverb pot with a 10k audio taper made a huge difference in the usefulness of the digital reverb by giving me better control over the exact amount mixed into the signal before the output tube.
After modding, I was pleasantly surprised with the sound, and look, of this amp. After a year playing it I also began experimenting with tube rolling, discovering that swapping out tubes, even of the same type, changed the character of this amp tremendously.
Weber 5F2H
After my initial experiences with the Bugera, I wanted something a bit larger and thought I'd try building an amp from a kit. I made this one as per the instructions initially, with one exception, an 8-position switch that allows selection of various plate voltages for the preamp and output tubes. This is a cool mod that allows more tonal flexibility. Working with the stock tubes, and with the plate voltage set at original spec, the amp had a great sound but was prone to feedback fairly early (around 4 on the volume pot).
After playing the amp for a year, and using a Kinder AFB+ with it, I began experimenting with swapping out the tubes (observing both tone and feedback changes). I tried various preamp tubes (both type and make) as well as a few different KT66 output tubes. I ended up keeping a NOS GE 12AU7A and a JJ KT66, for now....
Recently, following experiments with a Kay 703c (see below), I also removed one of the input jacks and replaced it with a pot for a variable grid stop. This is allows me to balance the input and the volume pot in order to get different tonal colors; I can even get the volume up to 12 without feedback now if I want (thanks to Weber for 1-upping Nigel), and without the Kinder AFB+. Of course, the A-weighted SPL is lower, but the timbral flexibility this offers is way cool!
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rogonzab
581 posts
Aug 12, 2014
6:11 PM
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Kay 703c (early 60's)
So after a new mod, and then a new build/mod, I was excited to work on a vintage piece of gear and settled on a little Kay 703c from the early 60s. This is also the only amp here with a pentode preamp. First, I replaced all of the caps and added both a 3-prong power cord and fuse for safety. Next, I replaced the tone and volume pots, increasing the volume pot to 1Meg. Finally, I made the grid leak resistor more appropriate for harp mics (both CR and crystal types) and sequentially decreased the grid stop resistors on each of the three inputs to give each a tonally distinctive character.
The first input drives the preamp the least (but is by no means clean): the second input distorts even more than the first and compresses when you push it hard: the third input is extremely distorted and compressed and can be driven into severe blocking distortion (actually a cool effect sometimes).
VHT Special 6
Finally, as the 6-Watt Special 6 was made with tinkering in mind, and since a lot has been written about this amp for harp, I couldn't help myself....
After much reading and listening to examples of the mods of others I decided to start with the Jim Rossen mods. The recordings here were made using those mods exclusively.
All of the amps so far have been single-ended and low-power, but great for learning and experimenting. A Bassman '59 Reissue is next though....
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