This is a study in the several methods Guild used in its '60s amps to generate the boingy boingy ... on-board reverb. Like Gibson, Fender, and Ampeg, Guild too produced a stand-along reverb unit; the RC20 which was modeled after the Gibson GA1RT; same tube lineup except with a solid state rectifier but this is a look at onboard reverb and specifically the several methods that were and are possible to couple the reverb tank to the electronic stage that is sending it a signal to be 'verbed.
There are two requirements; that the coupling device get rid of any DC so it doesn't appear at the input of the reverb can and matching the impedance of the reverb can with the internal impedance of the signal source; in the case of tube amps, the tube 1/2 that is 'sending' the signal. There are three ways to couple the tube section to the can; with a capacitor, transformer, or a speaker. In its '60s designs, Ampeg chose a capacitor ... shown in the red circle below and the reverb can in the blue box; two very successful Ampeg designs below (the dotted green circle on the left is the center third of a 6U10 tube - a three-section tube used almost exclusively by Ampeg):
The capacitor blocks the DC from the tube from appearing at the input of the reverb can and Ampeg specified reverb cans with about 2.5K input resistance which approximates the internal impedance of most signal tubes in common use (the resistance is set by the transducers that connect the can's wiring to the springs)
Fender and Gibson went a different route; they both use transformers to 'couple' the sending tube half to the reverb can. The transformer saves the cost of a capacitor because it blocks the DC and the secondary side of the transformer is 8 ohms .. the nominal input impedance of most reverb cans.
An early, if not the first, Guild amp with on-board reverb was the 98RT and it used an Ampeg-style capacitor to couple the drive tube to the reverb can:
In it's mid-60s T1 RVT and version 1 Thunderbird, Guild coupled the reverb can with a speaker whose impedance was already 8 ohms. It added a resistor to reduce signal strength and a frequency-selective capacitor:
At some point in the later 60s, the amp line was re-designed electronically and cosmetically. In the process, the speaker-coupled reverb was dropped in favor of the more conventional transformer-coupled design. Like a Silvertone 1484 and other Valco designs, Guild relied on a reverb 'tray' in the preamp chassis mounted spring-side up and not a can in the bottom of the cabinet (late 70s Fender design shown for comparison);
And finally two modern designs; Qvart's JCM 800 series Marshall 4100 on the left and JP's Mesa on the right. Instead of a transformer, the Marshall uses op amps / transistors to drive the reverb can (and op amps to recover the boingy). The Mesa uses a conventional transformer although JP reports that this design doesn't generate a 'verbed signal that is quite loud enough for him ... I suspect it has something to do with the relatively low 110VDC on the return side but, until the late 60s, I still thought the moon was made of green cheese.
There you have it; Guild tried it Ampeg style, then they tried it Burger King's 'Have it your way' style, but ultimately reverted to the more conventional transformer-coupled design however I don't think adopting transformers would have made much difference; they saw themselves - like Gibson - as a guitar company first and amps as accessories. Too bad.
There are two requirements; that the coupling device get rid of any DC so it doesn't appear at the input of the reverb can and matching the impedance of the reverb can with the internal impedance of the signal source; in the case of tube amps, the tube 1/2 that is 'sending' the signal. There are three ways to couple the tube section to the can; with a capacitor, transformer, or a speaker. In its '60s designs, Ampeg chose a capacitor ... shown in the red circle below and the reverb can in the blue box; two very successful Ampeg designs below (the dotted green circle on the left is the center third of a 6U10 tube - a three-section tube used almost exclusively by Ampeg):
The capacitor blocks the DC from the tube from appearing at the input of the reverb can and Ampeg specified reverb cans with about 2.5K input resistance which approximates the internal impedance of most signal tubes in common use (the resistance is set by the transducers that connect the can's wiring to the springs)
Fender and Gibson went a different route; they both use transformers to 'couple' the sending tube half to the reverb can. The transformer saves the cost of a capacitor because it blocks the DC and the secondary side of the transformer is 8 ohms .. the nominal input impedance of most reverb cans.
An early, if not the first, Guild amp with on-board reverb was the 98RT and it used an Ampeg-style capacitor to couple the drive tube to the reverb can:
In it's mid-60s T1 RVT and version 1 Thunderbird, Guild coupled the reverb can with a speaker whose impedance was already 8 ohms. It added a resistor to reduce signal strength and a frequency-selective capacitor:
At some point in the later 60s, the amp line was re-designed electronically and cosmetically. In the process, the speaker-coupled reverb was dropped in favor of the more conventional transformer-coupled design. Like a Silvertone 1484 and other Valco designs, Guild relied on a reverb 'tray' in the preamp chassis mounted spring-side up and not a can in the bottom of the cabinet (late 70s Fender design shown for comparison);
And finally two modern designs; Qvart's JCM 800 series Marshall 4100 on the left and JP's Mesa on the right. Instead of a transformer, the Marshall uses op amps / transistors to drive the reverb can (and op amps to recover the boingy). The Mesa uses a conventional transformer although JP reports that this design doesn't generate a 'verbed signal that is quite loud enough for him ... I suspect it has something to do with the relatively low 110VDC on the return side but, until the late 60s, I still thought the moon was made of green cheese.
There you have it; Guild tried it Ampeg style, then they tried it Burger King's 'Have it your way' style, but ultimately reverted to the more conventional transformer-coupled design however I don't think adopting transformers would have made much difference; they saw themselves - like Gibson - as a guitar company first and amps as accessories. Too bad.
