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Old 2nd October 2012, 05:32 PM   #11
Struth is offline Struth  Canada
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Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

As stated above, this is just a poorly drawn schemo with a mistake. Soldano's effects returns are always to the grid of a tube. As usual, grid-stops are left out where they are most needed.

If you wanted to feed a signal into a cathode, the grid must be biased and so must be the cathode. There has to be a bias path and capacitive coupling must be used as the cathode will sit at a volt or so above ground. if you tried a direct connection as the erroneous drawing depicts, you would lose half the signal unless the signal was very small. The signal source would have to be a very low impedance.

Reverb returns were done like this very rarely, but the tank output is typically 100-200R DC and is like having a transformer drive. The circuit still benefits from having conventional bias components added and AC coupling.

Have fun
Kevin O'Connor
londonpower.com
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Old 3rd October 2012, 07:18 AM   #12
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Join Date: May 2010
Location: Hawkes Bay
Here's another schematic from Laney.

Got some bias and capacitative coupling, but that low input is gonna have a very low (for guitar) impedance.

Cheers
JimG
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File Type: pdf Laney_LC15.pdf (78.5 KB, 22 views)
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Old 16th November 2012, 08:57 PM   #13
BFC is offline BFC  United States
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The Ampeg Reverberocket uses a grounded grid amp as a summing stage in an interesting configuration where the preceding (conventional) gain stage is coupled from it's cathode to the GG stage's cathode, and the grid is DC coupled to the output of the reverb recovery amp via a 15k reverb level pot. The phase relationship of the overall signal chain is such that the grid signal from the reverb return is additive to the "dry" signal on the cathode. The conventional stage also feeds the reverb driver amp from the plate side...all of which I thought to be quite clever.

The reason I found this thread is that I was curious about GG stages in guitar amps myself; I found it's the perfect opportunity in the R-12R for adding an effects loop, due to the low Z and the fact that the signal level is appropriate - just had to break the cathode-cathode connection and insert coupling caps and the loop jacks.

As a side note: the R12-R used DC coupling between the aforementioned cathodes (a 22k resistor) and this 50+ year old amp has it's share of noisy resistors. Adding DC blocking caps in series with the loop jacks served to eliminate some of the resultant noise, so that was a nice bonus and is probably a good practice for anyone considering using the cathode coupled GG arrangement as Ampeg did.
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