I've started on a 5W tube amp build and have kept things relatively simple so far, controls consist of gain, tone, and master volume only (first build). Basing the circuit off my Valve Jr. w/ mods to improve tone and whatnot. Will post pics once building begins, so far I've only simulated things (don't want to blow up fancy parts )
One personal touch I'd like to add is a channel switch that changes the overall amount of gain in the signal, e.g. separate "Clean" and "Dirt" channels. I've done a bit of research but I can't seem to find how such a thing is implemented electronically. My first idea has been to feed the guitar input into a transistor amplifier circuit before sending this output to the tube-driven preamp, where the switch can bypass this circuit for the "Clean" setting. Seems reasonable to me as people are glad to feed transistor-driven distortion pedals into tube amps, but I'm new to this and would like some opinions.
Welcome to the fold brother - this is the best guitar/amp forum on the interwebznet!!!
Sounds like a plan. A two channel amp needs two preamp sections and, as long as the specs on your transistor pre amp match, it can be done. I'm fairly new to amp repair but, there are several very knowledgeable amp builders, on GAB, who would love to see your schematics. Can you post schematics of the amp and transistor pre?
Walt wrote:But when the hour is nigh, and the lights are low, and I got a little toothpick of a shwag joint in my teeth, and my friends want to hear me play "Into the Void", or "TNT", "or "Cemetery Gates"...I plug my 600 dollar guitar into my 150 dollar amp, and I am a Rawk gawd.
Do you have a schematic of what you're intending to build? One cool trick to add an input boost is to use an LND150 MOSFET as a switchable boost stage. Some folks over on ax84 have played around with this.
Loop wrote:I’m currently shopping for a 1996 Red Dodge Viper with yellow wheels. Who gives a shit about taste?!
For a first build, I would keep it simpler: just put the preamp cathode bypass caps on a switch. With them out of the circuit, you'll have a clean channel; in the circuit is dirty.
Schematic attached. Very similar to the AX84 P1, though I may be recycling the transformers from my Valve Jr. thus the power supply and biasing will be different. Haven't decided on that one yet. What I described would be added at the "Vin" node.
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Zozobra wrote:One cool trick to add an input boost is to use an LND150 MOSFET as a switchable boost stage. Some folks over on ax84 have played around with this.
Link? The reading and schematics over there have been really helpful--the P1 is virtually the same circuit as the Valve Jr. with a tone stack.
jgreenwd wrote:For a first build, I would keep it simpler: just put the preamp cathode bypass caps on a switch. With them out of the circuit, you'll have a clean channel; in the circuit is dirty.
I do want to keep it simple, but the transistor stuff is a bit more familiar to me since I just finished studying electronics in undergrad--I'm completely new to tubes, part of the reason why I'm keeping the wattage low. Could you elaborate a bit on bypassing the cathode caps?
BroSlinger wrote:Pretty sure my Egnater seminar amp has a footswitchable boost that sounds like what you want. I could email you a schematic.
Would be appreciated! My go-to amp is the 15W Tweaker combo.
I think jgreenwd is taking about switching the cathode resistor bypass caps in and out of the circuit as they add around 6dB of gain when added and they can be chosen to boost at a certain frequency. The 22µF caps in your circuit will be pretty much full range boost. Lowering the value will cut bass from the boost. Ye typical marshall circuit uses 680n for a mid boost and tighter bass, which sounds more gainy than 1µF and larger.
Zozobra wrote: I think jgreenwd is taking about switching the cathode resistor bypass caps in and out of the circuit as they add around 6dB of gain when added and they can be chosen to boost at a certain frequency. The 22µF caps in your circuit will be pretty much full range boost. Lowering the value will cut bass from the boost. Ye typical marshall circuit uses 680n for a mid boost and tighter bass, which sounds more gainy than 1µF and larger.
Exactly. Ck in this picture is a cathode "bypass" cap. It bypasses the stage to AC signals for frequencies greater than (2*pi*Rk*Ck)...if I remember correctly.
Since you mentioned simulating, I'm going to assume you're familiar with spice. Try removing Ck from one (or both) preamp stages and look at the frequency response, versus as-is.
Insert your desired values in place of the 1u and 2u. I would start with 22u and 1u. That will give 2 obviously different levels of gain.
Neato frequency response below. Included 680p per Zozobra's comment on the Marshalls. Got me thinking about switching between the 20/.68u to adjust the tightness of the bass. Thanks to jgreenwd!
FreqRes 20 1 .68.PNG
Wayne wrote:Also Ibanez makes those Tube Screamer amps.
This might be what I'm aiming for. Add switchable distortion circuitry with separate controls so the volume of the two channels can be relatively equal in loudness. I've used the Tube Screamer Mini with my Valve Jr., it's a nice pairing. I wouldn't mind embedding the circuitry in the amp verbatim; there's a nice analysis over here: https://www.electrosmash.com/tube-screa ... #schematic
Any suggestions on reliable sites for tubes/transformers? I've bought from Mouser in the past for other projects, but it looks like they don't have tubes. They do have Hammond transformers, and I've seen that name in my research (again, I'm a noob) without fully understanding which are some of the better ones.
Are you going to stick with the P1 or are you looking to modify it into a P1x, HO, or SEL? That would change the transformer needs. As for tubes, you can find those at several different places; even Guitar Center.
i know this is slightly off topic, but the board in this link drops into a valve jr. ive built one with the jcm800 option and did a few mods. it was killer.
Last edited by shep on Sun Feb 04, 2018 8:10 pm, edited 2 times in total.
Penny wrote:Any suggestions on reliable sites for tubes/transformers? I've bought from Mouser in the past for other projects, but it looks like they don't have tubes. They do have Hammond transformers, and I've seen that name in my research (again, I'm a noob) without fully understanding which are some of the better ones.
i use mouser for everything i can and tube amp stores for everything else. ive used only hammond transformers from mouser so far. they work great for me.
jgreenwd wrote:Are you going to stick with the P1 or are you looking to modify it into a P1x, HO, or SEL? That would change the transformer needs. As for tubes, you can find those at several different places; even Guitar Center.
Sticking to the P1, open to modding in the future tho. Not sure if I'll be looking to mod further after completing or working on an amp with higher wattage.
long standing member wrote:Hammonds are top notch.
Tons of people love Edcor, probably cheaper.
Stewart-Mac has a house labeled brand, also well liked.
I'm a Stewmac fan. Will need to check those out.
shep wrote:i use mouser for everything i can and tube amp stores for everything else. ive used only hammond transformers from mouser so far. they work great for me.
Sounds solid. Will probably go this route.
Looking forward to getting back into working on this thing. New job's been taking everything out of me, but I'm starting to get settled.
jgreenwd wrote:Are you going to stick with the P1 or are you looking to modify it into a P1x, HO, or SEL? That would change the transformer needs. As for tubes, you can find those at several different places; even Guitar Center.
Sticking to the P1, open to modding in the future tho. Not sure if I'll be looking to mod further after completing or working on an amp with higher wattage.
I've used Matt's stuff before and it's solid. 113+shipping vs 85+shipping. I've also used Edcor and it's good, but their stuff is usually made to order. Also, the comparable parts are the same price as the Hammonds at the moment. Copper and steel aren't cheap right now. Triode electronics (Classic Tone) doesn't make a PT to that spec, that I can find.
If you were comfortable with it, you could use a Fender Revibe PT from [url]tedweber.com[/url] and a tweed champ OT, but they are both deviating from the original schematic a lot.
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