Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chrisq on April 25, 2013, 01:09:02 pm
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So... i have a 1484 that I love but... i play surf/garage stuff so I'd like a better reverb. I've seen some involved mods but...
This may be novice of me but, since it is a piezo reverb, couldn't one just basically put a 1/4 jack on the in and out, and place a powered reverb unit (i'e a little lanilei or vanamp) and use it as an effect send? just power it from the outlet on the back. I know that i could just use an outboard one without alteration... but... there is something about hiding it or having it self contained that is attractive.
I'm up for suggestions! I've already hung the spring upside down and changed the tube to a 12DW7 tube that helped a lot but it still lacks depth.
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This might work?
With respect,Tubenit
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There may be some problems with impedance and the single size/voltage converting it to an effects loop just by plumbing 1/4" jacks in on the in/outs.
impedance:
the "Send" output impedance is ??? (how do you figure that? (someone?) they used that 6CG7 as a para-phase PI to drive the reverb box. (is that R47+R48? 1068K + the 27K R46 plate on the other triode
on the "return", the impedance is about 600K? (that'd be ok) but it goes through a 560K/68K voltage divider (R40,R41), thus attenuation is 10% (may or may not be a problem).
signal voltage:
the first 12ax7 triode it hits is a 100K plate, 2.2K cathode gain stage that'll ramp up the input @ maybe 60Mu, then on to the paraphase 6CG7, which might might be good for 10Mu. If the effects box is expecting instrument level inputs, you might encounter a problem. Given the 10% attenuation on recovery, the upfront gain of the driver might work out ok.. you might only get 10% travel on that "REV DEPTH" control before the 'send' signal is too hot...
for my first attempt at, I'd replace that R40 560K with a 1M or 500K pot and be ready to dial in the signal level, and start with both pots (the new return trim pot, and the REV DEPTH at "zero") The reverb signal is mixed back into the signal destined for the power amp with the plate R's R24,R25, and R26 with the reverb signal getting a pretty small piece ( 560K vs. 68K and 68K ).
another option:
You've got two noval tubes to play with, if you want to gut the existing reverb circuit, you can rewire those sockets to be a nice buffered effects loop. those 1484's aren't exactly friendly to a lot of re-engineering. you might clear enough space to install a small eyelet/turret board on 1" stand-offs to put a new circuit in there.
http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf (http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf)
Of course, that's a lot more work than adding a couple 1/4" jacks. With all that, you'd probably want to skip it and put it out front of the amp (guit->effect->amp)...
another option:
You've got two tubes in the reverb section, so any reworking on the circuit to handle a spring reverb could be done with both tubes. Ampeg and Magnatone both used tubes to drive and to recover a Hammond reverb tank. (an external enclosure with a tank in it?)
thats probalby too much work, and plus you'd have a reverb tank to deal with externally...
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another option:
You've got two tubes in the reverb section, so any reworking on the circuit to handle a spring reverb could be done with both tubes. Ampeg and Magnatone both used tubes to drive and to recover a Hammond reverb tank. (an external enclosure with a tank in it?)
thats probalby too much work, and plus you'd have a reverb tank to deal with externally...
I've done this to my brother's 1484. It isn't easy, but can be done. A new head box will have to be made so you can mount the reverb pan, and reorganizing the preamp tube order is a good idea so the signal doesn't go back on itself. It IS a lot of work in these amps, but sounds good and works fine once done. On my brother's I added a Fender reverb circuit and pan. Before I did this, the reverb didn't work and the Silvertone pan was missing so anything was an improvement. The amp did sound and play fine without reverb too though.
Greg
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Sometimes it's more complicated and easier to just use an external reverb. The amp is what it is