ldk
Joined: Nov 08, 2017
Posts: 368
San Francisco Bay Area
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Posted on Jun 13 2023 11:19 PM
I've rebuilt some vintage tube amp record players and made some guitar amps: three single-ended Champ type and one push-pull Deluxe type.
I recently got a mid-50s Silvertone tape recorder with a nice single-ended amp and poor mechanism. I didn't want to make another SE amp, so I thought I'd make a 6G15-like reverb unit, which is essentially a SE amp with an extra tube. I've based my design on the schematic here: https://www.mojotone.com/amp-kit-resources. Most 6G15s have a solid state rectifier, but I'm going to use the tape recorder's tube rectifier. Otherwise, it's like the schematic.
I've bought a number of parts (choke, 12AT7 tube, pots, caps, resistors) and have a copper box from a previous project. The rest is from the tape recorder.
Here's the outside; I like that the tubes are visible.
It's only a 12" box, so I'll use a short pan, but I'll have some jacks on the rear to hook up a 17" pan.
Here's the inside with the heaters wired up. Lot's more soldering to do.
Apart from the spring pan, it's about a $120 project. I'll let you all know how it turns out.
— If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.
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da-ron
Joined: Jan 02, 2009
Posts: 1307
The original Plymouth, UK.
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Posted on Jun 14 2023 02:46 AM
Nice looking project.
I went the other way and turned an old 6G15 project into a VibroChamp, using a chassis and cab from an old Peavey Backstage.
It sounds fantastic. I just love the tremelo on it
— http://thewaterboarders.bandcamp.com/
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DeathTide
Joined: Apr 13, 2018
Posts: 1378
New Orleans
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Posted on Jun 14 2023 08:44 AM
Looks awesome! I'm super curious if the tube rectifier makes a big difference.
— Daniel Deathtide
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terma_reverb
Joined: Apr 17, 2012
Posts: 47
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Posted on Jun 14 2023 01:34 PM
DeathTide wrote:
Looks awesome! I'm super curious if the tube rectifier makes a big difference.
+1 on this by me. You could easily experiment using something like this and let us know about the findings! Cool project overall!
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ldk
Joined: Nov 08, 2017
Posts: 368
San Francisco Bay Area
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Posted on Jun 14 2023 06:49 PM
terma_reverb wrote:
DeathTide wrote:
Looks awesome! I'm super curious if the tube rectifier makes a big difference.
+1 on this by me. You could easily experiment using something like this and let us know about the findings! Cool project overall!
Good suggestion on doing a quick replacement of the rectifier tube with a solid state version. I'll plan on doing that.
From what I've read about solid state vs. tube rectifiers in amps, the tube rectifier can be less response to current demands, leading to 'sag' and 'sponginess,' which may be good or bad depending on the player. I suspect the difference will not be too great for a 6G15. I'm guessing that solid state was used in the original 6G15 for cost reasons.
— If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.
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ldk
Joined: Nov 08, 2017
Posts: 368
San Francisco Bay Area
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Posted on Jul 30 2023 04:55 PM
I've finished my reverb project and finally recorded some guitar through it with the tube and with the solid state rectifier. Because there's a voltage drop of 20 V with the tube and only 1 V with SS, I thought there might be a volume difference, but not much else.
You can find the samples here.
https://drive.google.com/drive/folders/17CdX5OGB4PCDsIKqfCK9sXNIbILSeLIj?usp=sharing
(I made the folder readable by anyone with the link, so if you have any problems, let me know.)
All were recorded with the 'Input.mp3' file through a Surfy Pan with settings of 6, 6 and 6 for dwell, mix and tone. First is an E chord left to die out, then individual strings played, then the E chord stopped after about 1/2 a second.
To really ascertain the differences, put all the files in your favorite DAW and loop through a little section. There is a ~0.5 dB increase in volume in the SS files. Interestingly, there are high frequency overtones in the tube samples.
I don't know what differences I'll hear when I play a song with the SS rectifier vs the tube rectifier. We'll see.
— If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.
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DrippyReverbTremolo
Joined: Mar 11, 2023
Posts: 144
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Posted on Jul 31 2023 05:14 AM
Cool. Impressive too. I'm handy but know enough about tube amps to know that I don't know enough to work on them.
Last edited: Jul 31, 2023 05:14:38
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edwardsand
Joined: Jun 29, 2018
Posts: 769
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Posted on Jul 31 2023 07:57 AM
Nice. Since I've been working on building a reverb unit, a lot of the details are fresh in my head. If you are using a plug-in SS rectifier, I assume that's a full wave bridge rectifier as opposed to the half wave that was used in the original 6G15; it would give higher voltage than the half wave and tube rectifier setups, like you measured. The only concern would be if you were using a 6K6 tube, since those don't like very high voltage, as compared to a 6V6.
Also, what were the specs on the original output transformer? How close are they to the 6G15 reverb transformer specs?
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ldk
Joined: Nov 08, 2017
Posts: 368
San Francisco Bay Area
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Posted on Jul 31 2023 02:28 PM
Here's the finished product. The SS rectifier is the yellow glowing piece near the power transformer. It's a full-wave rectifier that has two 1N4005 diodes, with an LED+resistor connected to the filament power, all in the bottom of a pill bottle filled with epoxy. The reverb has a 12AT7, a 12AX7 and a 6V6. The rectifier tube, not pictured, is a 6X5.
I used the output transformer from the tape recorder, so I don't know its specs. I used my son's LCR meter and found the load the 6v6 sees is about right.
— If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.
Last edited: Jul 31, 2023 19:29:07
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edwardsand
Joined: Jun 29, 2018
Posts: 769
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Posted on Aug 01 2023 10:36 AM
What does that third 12AX7/12AT7 size tube do?
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ldk
Joined: Nov 08, 2017
Posts: 368
San Francisco Bay Area
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Posted on Aug 01 2023 12:00 PM
edwardsand wrote:
What does that third 12AX7/12AT7 size tube do?
I wondered if anyone would see that there was an extra tube.
The tape recorder came with two 12AU7s and the copper box had an extra hole. Rather than just cover the hole, I put in one 12AU7 and just hooked up the heater for extra glow.
While I'm posting, I'll describe the switches. The top ones are for power and bypass, which grounds the pan output. (I should have used stomp switches.) Also, I noticed that on bypass the guitar signal lost top end, unless mix was on 1. So, for fun, I put a switched treble bleed on the mix pot. That's the toggle switch on the front between the mix and tone knobs. I haven't really explored the use of this switch. One thing is that when it's engaged the dry signal is greater, and the mix has to be turned up to retain comparable reverb.
Overall, it turned out well, and it's good to have a tube reverb to play around with.
— If I'd stop buying old guitars to fix, I might actually learn to play.
Bringing instruments back to life since 2013.
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