Help with Preamp Mods

[tonefiend]

Hello, Im looking for some help with some preamp mods to my amp head. Its an old montgomery wards gim 9151A, I have attached the schematic for your reference....

In looking at the schematic there is no cathode bypass cap on v1a? hmmmm Also the cathode bypass caps that are present on the preamp are 35uf 6 volt caps? I dont have the math skills to figure out if that cap is blocking any guitar frequencies or letting them all through?

any help would be much appreciated

[eschertron]

Hello, Im looking for some help with some preamp mods to my amp head. Its an old montgomery wards gim 9151A, I have attached the schematic for your reference....

In looking at the schematic there is no cathode bypass cap on v1a? hmmmm Also the cathode bypass caps that are present on the preamp are 35uf 6 volt caps? I dont have the math skills to figure out if that cap is blocking any guitar frequencies or letting them all through?

any help would be much appreciated

Thanks for posting the schematic. So without a cathode bypass on V1-A (input two) is it discernibly lower gain than input 1? That's the reason I can imagine for the difference, having two ever-so-slightly different channels.

the voltage rating on the 35uf/6v caps is a little funky, but should not affect the freq response of the circuit. They must have gotten a good deal on those...

Is this amp a loud, clean monster? What do you want to do with it?

[Alan0354]

The main question you did not say is what do you want the mod to do?

[tonefiend]

Thanks for posting the schematic. So without a cathode bypass on V1-A (input two) is it discernibly lower gain than input 1? That's the reason I can imagine for the difference, having two ever-so-slightly different channels.

the voltage rating on the 35uf/6v caps is a little funky, but should not affect the freq response of the circuit. They must have gotten a good deal on those...

Is this amp a loud, clean monster? What do you want to do with it?

Hello, thanks for your response, Im really wondering how the 35uf/6v cap is affecting guitar tone... I know fender uses 25uf/25volt cathode bypass caps... just wasnt to sure what the 35uf was doing to the guitar signal, as this im pretty sure was a designed as a Bass amp.... so I wanted to turn it into a guitar amp. I have got it working, and connected it to an 8ohm single 12" closed back guitar amp yesterday, it sounded pretty good, but bass frequency did seem more prevalent.... of course we have to consider that I was going through a closed back cab as well, with a celestian 80watt speaker..... anyhow im looking for a bit more chime... would lowering the cathode bypass cap give more hight end chime? Also, im going to build my own 1x12 cab.. solid pine, with wooden baffle, no pressboard for me thanks... Also probably going to go with an 8ohm Alnico 12" speaker ;-)

[Enzo]

The cathode voltage on these first stages are typically only a volt or two, so a 6v cap is more than enough. Back then, a 6v cap cost less than a 25v cap.

The difference between 35uf and 25uf is not enough to talk about.

One of the most popular guitar amp heads of all time is the Fender BASSman.

Try the amp through different speakers. That would be my first response to "too bassy" or "not bright enough."

[tonefiend]

The cathode voltage on these first stages are typically only a volt or two, so a 6v cap is more than enough. Back then, a 6v cap cost less than a 25v cap.

The difference between 35uf and 25uf is not enough to talk about.

One of the most popular guitar amp heads of all time is the Fender BASSman.
ry
Try the amp through different speakers. That would be my first response to "too bassy" or "not bright enough."

Thanks for your reply... How about the missing cathode bypass cap on V1a? Definately going to try different cabs...

[Tom Phillips]

... just wasnt to sure what the 35uf was doing to the guitar signal, as this im pretty sure was a designed as a Bass amp.... so I wanted to turn it into a guitar amp...

Basically using a 35uF cathode bypass cap in that circuit will allow the tube stage to pass all the guitar frequencies. If you plane to overdrive the amp then it will probably sound muddy and one way designer's address that is to limit the low frequency content passed by the preamp. And...one way to do that is to lower the value of the cathode bypass cap. It's all explained very well in section 1.18 of the sample Chapter 1 of the book Designing Valve Preamps for Guitar and Bass, Second Edition posted at http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
Give it a read. Lot's of good figures and detailed information. Better than us trying to cover it with a forum post.

[Enzo]

Also consider it is a major difference from the other channel. You want to hear its effect, try playing through the other channel which lacks it.

[Alan0354]

I don't like both my Celestion G12T-75 and G12H-100. They both have booming bass that sounded ugly. I think your problem is mainly the speaker. I like open cabinet with American sounding speakers.

As for 35uF, it does not matter in your case. You can change to 22uF, it won't change the sound much. You'll have more gain if you put a 22uF cap to bypass the first stage. Try it and listen.

[sgelectric]

Are R3 and R35 values unusually low for guitar preamp circuits?

SG

[Chuck H]

Yep. 50% voltage division going on there. Increasing them substantially would also decrease the HF roll off frequency of the parallel caps though. So if those R values are increased the cap values need to be decreased.