| Author | Message | ||
| Ian Azrikan (inertian) Username: inertian Registered: 10-2013 |
My bandmate was so impressed with the sound of my restored 110 RD 50 (thanks to the generous help of Terry Loose and others on this board) that he bought a 1978 112 RD Sixty-Five and asked me to look it over. Its a GD-1 board, running EL-34s. Sounded fine except the first set of Gain/Volume controls seems to produce a much lower level of sound than the second set of Gain/Volume controls with the same settings. For example, to get both "channels" to achieve the same perceived loudness, the first set of controls would be Gain=5, Volume=10 vs the second set of controls at Gain=0, Volume=10. Did a complete cap job (the power filter caps had physical signs of deterioration) and rebiased, the amp sounds great but the volume disparity continues. Noticed that if both Gain/Volumes were set to 10, they sounded identical in loudness (switching between them with the footswitch). Further testing (including IC swapping, which had no effect)indicated that the first Gain control seems to have minimal impact until about 7 on the knob, and then quickly increases until 10. Question 1 - the schematic shows a 1M audio pot for the first Gain control, the amp has a 1M reverse audio pot installed. Which is correct? Question 2 - Besides a wrong or bad pot, what else could be the cause of this behavior? Thanks for any guidance. | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
The GD-1 circuit with the mechanical relay is somewhat crude compared to the later versions. The schematic shows 100k reverse audio taper and 1M audio taper for the gain pots (the former being the hotter channel). Who knows - maybe your particular amp was fitted with two reverse audio pots? MM schematics are not always 100% accurate especially if you have an early version of a product (which a '78 (silver badge?) RD certainly is). My advise is to experiment with different pot values (and tapers). In the end you'll never get the same distortion effect out of this version compared to GD-2/GD-2A. That doesn't make it any less of a cool and rare amp. Cheers, Lars Verholt | ||
| Ian Azrikan (inertian) Username: inertian Registered: 10-2013 |
Thanks for the response. The amp is indeed a silver badge model, board dated 10/22/78, with 11/1 written on one end of the chassis. All the other pots match the GD-1 schematic and measure in spec, including the 1M reverse audio first Gain pot. I'll try to locate 1M and 100K reverse audio pots to see which works better. Since this is a single channel amp and the second Gain pot (100K reverse audio) work perfectly, it seems logical that reverse audio is the correct taper. Meanwhile, the later GD schematics show both Gain pots as 100K reverse audio. If anyone has a similar amp and can verify what value/taper pot is in the first Gain position, it would be most appreciated. | ||
| Ian Azrikan (inertian) Username: inertian Registered: 10-2013 |
After trying a bunch of different value and taper pots, I located a 100K reverse audio pot and it works perfectly in the first Gain control position. Both sets of controls now behave identically, so you can easily dial up clean or dirty on either "channel" at any volume with the same settings. I generally like to follow the specs the designers laid out, but clearly something was not correct here based on the amp's behavior when it was brought to me. Suppose its possible the schematic is incorrect, as it was hand done early on, and all the following revisions use a 100K RA pot for Gain control in both sections. Anyway, the amp is going back to its happy owner tomorrow. |
Tue, 03/25/2014 - 09:54
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