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Silvertone guitar amp alteration.

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Posted on January 25, 2021 at 20:07:21
RonRon
Audiophile

Posts: 43
Location: TX
Joined: January 16, 2021
 I have a Silvertone model 1449 guitar amp. It has two 12AX7's and a 6V6GT power tube with a 6X4 rectifier. The amp has a tremolo which runs the strength off one side and the speed off the other side of one of the 12AX7's. This is an extremely clean sounding amp, even wide open. Hardly any break-up at all. So since I never use the tremolo, I'm wondering if I could just add 1/2 or a whole 12AX7 gain stage and use one of the trem pots to control the new gain stage to overdrive the amp. I hate using a solid state overdrive pedal and I've made several low voltage tube overdrives that just don't break up nicely at all. Any advice would be much appreciated.
R.W.R.

 

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RE: Silvertone guitar amp alteration., posted on January 25, 2021 at 22:08:38
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 Being as you have two 12AX7s to play with, you could copy the front end of the Marshall high-gain "master volume" series (JCM800 model 2104/2204 or 4010). The schematic is here:

https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM800-50W-4010-Schematic.pdf

There's a detailed description of this circuit at the Website below. Scroll down to the subheading "The Birth of the High Gain Amplifier." I'm in the process of building this now for my first DIY guitar amp.



 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 08:27:07
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 The schematic shows unbypassed cathode resistors for both halves of the preamp tube. The cathode resistor for the input stage is 4.7K. The second stage uses 2.2K.

That might have been a cost-saving measure, or maybe to limit the gain intentionally so kids would not blow speakers.

Adding a 22uF/25V capacitor across those resistors will give you the gain you're looking for without the need for major modifications.

 

Excellent idea!, posted on January 26, 2021 at 09:08:38
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 And to go a step further, if tremolo is not needed, he can also bypass the 6V6 cathode. This will all make a big difference in gain with very little work.







 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 13:52:29
RonRon
Audiophile

Posts: 43
Location: TX
Joined: January 16, 2021
 I'm wondering what kind of caps would be appropriate here. All I have on hand are polarized electrolytics in the correct capacitance. When you say across the resistors am I correct in assuming parallel with the resistor in series with the line? As you can tell, I'm not expert, and just want to make sure before I do it.
R.W.R.

 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 15:13:14
dee eye why
Audiophile

Posts: 1148
Location: so. ohio
Joined: March 20, 2003
 Polarized electrolytic are exactly what you want to use. You are correct that the cap is in parallel with the resistor.


.
Freak out...Far out...In out....

 

Watch the polarity!, posted on January 26, 2021 at 15:52:10
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 The + side to the tube cathode (pin 3 and pin 8), negative to ground.

 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 19:13:29
RonRon
Audiophile

Posts: 43
Location: TX
Joined: January 16, 2021
 OK, I just finished installing the two bypass caps. The amp is definitely louder and also seems more sensitive but is still clean unless I strike the strings really hard with the volume all the way up. Other members in my household don't have the same appreciation for loud music as I do so I was hoping for less headroom, just the opposite of what most players are looking for. At any rate the amp is improved and much thanks for all the advice. Guess I'll try tweaking the 6V6 next, and if that doesn't do the trick I'll add the Marshallish gain stage.
R.W.R.

 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 20:25:20
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 what's the plate load resistor of each halves of 12ax7?

 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 20:36:59
zarniwoop
Audiophile

Posts: 590
Location: NY
Joined: November 28, 2002
 you should be able to get a pretty substantial breakup (still fairly loud for others in nearby rooms) with this topology. the simplest circuit w these tubes is the fender champ, using only 2 12ax7 gain stages and se-6v6 out.

i see some slight weirdness in the silvertone circuit that may account for lower gain. as mentioned, the preamp cathode resistors should be bypassed for more gain. also, the 6v6 cathode resistor should be bypassed similiarly. they're doing something weird by sharing a cathode resistor from the tremolo for some reason, and having the sum total cathode resistance left to generate local nfb. and overall, no global nfb. maybe this was to keep the amp cleaner..? but there is a reason champs are very popular still today and silvertone amps are less-so.

personally, i'd scrap this circuit and modify it to a vibrochamp, which has really nice sine-wave LFO tremolo and plenty of gain to breakup when modestly cranked up. or you could add a pure dirty 12ax7 section like the 80s marshall preamp if you don't use the tremolo. but i see this as basically the same as using a solid state overdrive pedal.

 

RE: Silvertone guitar amp alteration., posted on January 26, 2021 at 21:10:21
rivervalley817
 the nice thing about a guitar pedal is that you can turn it off

a clean tube based platform is great to load with a tube based pre-amp

Behringer has one for just $120.00 with three channels

put a compressor ahead of it or after it, slam the amps front end and learn some licks ... turn it off and do some chord leads like a jazzbo ... you can get tones at lower volumes with a tube pre-amp too!

slinging solder can be fun, but a plug and play solution lets you ... play

regards,



 

RE: Silvertone guitar amp alteration., posted on January 27, 2021 at 07:36:25
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 if I remember correctly, i think the tremelo input is sampled from UNbypassed 6v6 cathode and pumped into trem circuit.

 

RE: Silvertone guitar amp alteration., posted on January 27, 2021 at 12:32:01
RonRon
Audiophile

Posts: 43
Location: TX
Joined: January 16, 2021
 The plate load resistors are 220k for he first half and 100k for the second.
R.W.R.

 

RE: Silvertone guitar amp alteration., posted on January 28, 2021 at 07:47:03
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 you could try a 220k instead of the 100k on the second half.

it won't make as much difference as bypassing the cathodes, but you may just eke out enough gain to make it a little crunchy with pick attack.

 

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