5 Watt Amp mods

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5 Watt Amp mods

Postby Turnerguitar » 16 Sep 2014, 20:10

Got a hold of an Ampeg GVT5 and the schematic and it resembles a blackface champ pretty closely besides the tone stack. So, I'm wondering if anyone has any suggestions about mods. I would love to put a brownface or blonde bassman tonestack in it, however, I'm not sure about insertion loss with those tone stacks. Any mod ideas are welcome from simple tone stack swap to transformer swap to adding another gain stage, lets hear em! Also, if anyone wants to know plate voltages and what not, I can get those as well. The amp just doesn't sound amazing, however, I think it could without a whole lot of work.
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loudstroud (21 Sep 2015, 04:08)
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Re: 5 Watt Amp mods

Postby Turnerguitar » 16 Sep 2014, 20:36

By the way, that is a 6V6 instead of el84 :)
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Re: 5 Watt Amp mods

Postby sinner » 16 Sep 2014, 20:48

IMO relatively clean sounding amps need no mid control, but if you want it then it can be done without changing existing Baxandall tonestack.

Google bridged-t mid cut control
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Re: 5 Watt Amp mods

Postby Turnerguitar » 17 Sep 2014, 02:55

Yeah I'm not certain it needs a mid control either. The reason I'm trying to mod it is at clean levels, it sounds like a fairly nice solid state amp, however, when I push it into saturation, it gets a buzzy, fuzzy sound. I would even take fizzy overdrive over the drive that I'm getting from it now. If those adjectives mean anything to anyone :D

The reason I was thinking about putting the blonde bassman tonestack in it, is from what I've listened to those amps, they have a nice little mid scoop but also a mid frequency resonant peak that to me sounds vocal (however, I might be completely off). Do the brownface tonestacks have less gain loss than blackface?
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Re: 5 Watt Amp mods

Postby sinner » 19 Sep 2014, 22:48

Before you take a step on the tonestack try to change C6 for something smaller. Try with 22nF or even down to 4n7. 100nf coupling cap sounds like overkill here. I don't know, maybe it's normal with Baxandall but with more common tonestacks to big coupling cap is the one to be blame for fizzy overdrive tone - to much low FQ's being distorted

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Re: 5 Watt Amp mods

Postby sinner » 19 Sep 2014, 22:57

BTW

R5 is really what it says on the schematic?
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Re: 5 Watt Amp mods

Postby Turnerguitar » 19 Sep 2014, 23:58

Okay I'll change C6 and report back. Thanks for pointing out R5--it is NOT 1.5k--I copied that down wrong. It is 100k :) Any other values that look out of place? After I change C6, I'll test the plate voltages. Any other voltages that I need to test that might clue me into anything?
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Re: 5 Watt Amp mods

Postby Turnerguitar » 20 Sep 2014, 19:57

Wow, that cap change took the buzzy overdrive away completely! However, the clean tone is still completely uninteresting. It sounds like a decent solid state amp.

While I was inside, I checked plate voltage and calculated plate dissipation. Seems like all the voltages are the same as on the Fender Champ Schem. So, the 6v6 plate voltages are around 357v. I calculated the plate current at 40.2 mA and the plate dissipation at 14.358 watts which seems a little over biased... Any thoughts?
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Re: 5 Watt Amp mods

Postby sinner » 20 Sep 2014, 22:36

Nice :)

To be honest I don't think you'll squeeze much headroom from SE amp. What you can try is play with cathode resistor plus bypass cap values in the first gain stage. You can also try to apply some Nfb just for the hell of it - it's really simple. Another thing you can try is that bridged-t mid mod I have mentioned but make it fixed. After C6 and before tonestack place small cap (500pf?) in parallel with two resistors (470k?). Between those two resistors place small cap (1n) to ground. Play with values. The ones I give you if I remember the numbers right make SRV mod in that famous tweed covered SLO100
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Re: 5 Watt Amp mods

Postby DrNomis » 21 Sep 2014, 09:21

What about modding it to AX84 High Octane specs to get more gain out of the amp?, you might need to make another hole or two in the chassis for the extra triode stages..... :thumbsup


Note: You should be able to use the stock Power, and Output Transformers as well as the 6V6 tube....... :thumbsup


http://www.ax84.com/hioctane.html
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
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Re: 5 Watt Amp mods

Postby MoonWatcher » 21 Sep 2014, 13:28

sinner wrote:To be honest I don't think you'll squeeze much headroom from SE amp. What you can try is play with cathode resistor plus bypass cap values in the first gain stage. You can also try to apply some Nfb just for the hell of it - it's really simple.

Those are two things that I'd really concentrate on.

I typically will just remove the cathode bypass caps altogether and give that a listen. For the value of the cathode resistors themselves, I find that 1k5 will actually give the best overall performance. I may try to add bypass caps back in, but usually throw them on a switch.

The global negative feedback thing can also usually help, too. I like to start with a lot of feedback, just to find out how much I can clean things up. The benefit of putting in feedback is that it can be done with a pot, and then there's a presence control, too.

The downside of negative feedback is that it's generally not doing anything when the amp is cranked up, and most little amps tend to only sound good at or near max volume.

Aside from that, if the amp has the supply voltage dropped quite a bit in the B+ rail, there may be an opportunity to swap out something like a 10k to a 4k7 or even 1k - that will help a bit. Just be mindful of the voltages.

I think that on a lot of the current little amps tend to add grid resistors to v1b where you wouldn't see that with something like a Champ - even just a 1k resistor can help a bit.

A change that's usually profoundly for the positive needs to be determined if it's worth the cost - bigger output transformer. Most of these little amps come with a tiny one. A swap for a big sucker will almost always give you a lot more bottom end, and it will take away those really high nasty frequencies that are like chewing on aluminum foil. It will also usually be 25% the cost of the amp itself, maybe more. Some of them won't just drop in, so the chassis will need to be modified, there may be clearance issues (it might be too tall and hit the speaker), and there might be different output impedance taps than the original.

I guess the other one that's already been talked about is making the caps off the plates a bit smaller than someone might think would be very good sounding. I've seen guys drop them down to 10nf or even smaller.

Also don't want to rule out trying a different speaker - lots of replacements that are much more efficient than stock ones. I think little single-ended amps are probably best as heads - put one into a decent cab and it's really cool how big the amp can sound.
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Re: 5 Watt Amp mods

Postby sinner » 21 Sep 2014, 13:36

Gain? You mean drive? You can add more drive to the circuit with out extra gain stages

With extra valves you can go as wild as you can. As my understanding goes, OP is more after the tone than amount of drive

I was thinking more about it today anf really - bit of NFB with fixed resonance/depth mod would make the cleans lush. Amount of drive would decrease a bit, but the headroom would increase VIRTUALLY

Let me know if you need instructions how I see it
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Re: 5 Watt Amp mods

Postby sinner » 21 Sep 2014, 13:41

Cool post MoonWatcher. Personally I prefer to have bypass cap in the first gain stage. I dont care much for it in the last one
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Re: 5 Watt Amp mods

Postby Turnerguitar » 21 Sep 2014, 17:44

Yeah, you're right sinner--I'm mainly looking for clean fender tone of the blackface or brownface variety. I already have my drive amp so this one was supposed to be my clean amp.

I understand the NFB, however, I'm not sure what you're referring to by fixed resonance/depth mod :scratch: I'll do a little research on that
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Re: 5 Watt Amp mods

Postby sinner » 21 Sep 2014, 18:12

Fixed depth mod is just a resistor (100k) with bypas cap (4n7) after NFB resistor to OT secondary

To add NFB in SE amp cut that bypass cap on Vb off, set the NFB resistor of your choice from Vb cathode with or without depth mod to OT secondary of your preference

OK, this time all above is just a theory, so maybe before you do anything wait until other people comment on this
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Re: 5 Watt Amp mods

Postby sinner » 21 Sep 2014, 18:20

One more thing. According to M.Blencowe NFB applied this way is good to go, depth is just a simple mod and I see no reason why this would not work. If you get parasitic oscillation swap the OT's primaries around
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Re: 5 Watt Amp mods

Postby Turnerguitar » 21 Sep 2014, 18:46

Okay I will give this a try.

Am I only able to wire the NFB and fixed depth mod to one tap of my multi-tap OT? What are the effects if higher/lower NFB resistor values (where should I start--2.7k?)
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Re: 5 Watt Amp mods

Postby sinner » 21 Sep 2014, 20:17

I would say more in 27k to 100k range really :-)
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Re: 5 Watt Amp mods

Postby Turnerguitar » 21 Sep 2014, 20:22

Okay, do the higher values mean more smoothing or less smoothing? I'm thinking higher values mean less signal placed back into the circuit therefore less smoothing...?
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Re: 5 Watt Amp mods

Postby MoonWatcher » 22 Sep 2014, 02:00

Turnerguitar wrote:Okay, do the higher values mean more smoothing or less smoothing? I'm thinking higher values mean less signal placed back into the circuit therefore less smoothing...?

Higher values of resistance means less negative feedback. The more you lower the resistance, the more you allow for a signal to be fed back.

100k tends to be the max that you'll see with amps like the master volume-era Marshalls.

The plexi-era Marshalls have 47k, but I wouldn't say that it's incredibly more significant of negative feedback. I think a few of the Soldano amps actually use the 47k.

Other than the tweed Bassman, the brownfaced Fenders were really the first ones to commonly have negative feedback, and I think the brown Deluxe had something like 27k.

I wouldn't refer to it as smoothing as much as something like "dampening." It may help to do some research on it - there might be better expressions for what it actually is, and what it isn't.
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