Amp Maker SE5 extra gain stage

[Willholmania]

Hello all

I've searched but can't find anything on this so;

has anyone built one of Amp Maker's SE5s? I have and I've added a tube driven reverb section, plus a VVR kit from Barry at Amp Maker. I'm now keen to add a switchable gain stage but I'm undecided as to where in the circuit I should add it. I'm erring towards as cascode boost circuit right at the front of the amp to boost the input, simply because I love the sound of it and want to be able to preserve the current signal path (the extra gain stage would be foot-switchable).

I just wondered whether anyone here has built one of these amps then added a gain stage?

Thanks

Will

[Willholmania]

Okay. Well I've planned a layout for the cascode boost section so if anyone's interested shout and I'll let you know how it works out.

[pdf64]

Yes, I'd be interested to know what circuit you've used (both for the cascode and switching) and how it works out.
There should be a lot of gain available from a cascode; without some attenuation and tone shaping (ie rolling off top and low end) it may be too much.
Good luck - Pete

[Willholmania]

I'll attempt to attach the circuit I'm nicking (from AX84). For switching I'm using one of Weber's channel switch kits. It runs off the heater circuit.

I won't get time this week but hopefully I'll get it done within the next few weeks.

[Willholmania]

Just got this done and it works a treat. I used a little relay switching board from Weber in the States (I've had it for ages and it was only 10 bucks but unfortunately Weber charge a lot for posting to the UK). I've wired this up via a push-pull pot on the amp's gain control and also to a jack socket so I can use a footswitch with it too. It gives just the right amount of extra gain and makes a great amp into a superb one, in my humble opinion. Here are some pics. They're not great (sorry). It's hard to pick out the extra boost stage as it's got pretty crowded but the reality is there are only about 10 extra components.

[Willholmania]

Hi. I spoke too soon. I have a loud buzz which only goes away if I turn the gain right down to two or three. I'm guessing it's some kind of oscillation. Here's the original circuit that I've bolted the new stage on the front of, (minus the 1m resistor which sits in front of both). Can anyone see an obvious problem?

[pdf64]

There seems to be 3 cascading gain stages on the same power supply node?
It would be a good idea to add another decoupling node, 10k and 22uF fed from point C, for the cascode stage.
Pete

[Willholmania]

Hi Pete. Sorry - I should have included that in the description. That's almost exactly what I've done. I have a10k and 47uf from point C, feeding the cascode.

Do you think replacing the cheap lapped screened cable I've used on the inputs with better quality braided screen wire could help?

[pdf64]

I doubt that the root cause is due to poor internal cable, though it may be a contributory factor.
The general topology have having a switchable gain stage at the input is not ideal, it may be better to switch a later stage.
Can you determine whether the buzz is due to electronic or acoustic coupling? eg if the master vol is turned very low and the amp chassis is physically separated from the speaker cab, is the buzz affected?
Do any of the tone controls affect it?
Have you got use of an oscilloscope?
Pete

[Willholmania]

Unfortunately I don't have a 'scope. I'm fairly sure the problem is electronic rather than acoustic. What I don't understand is why the cascode section works in the AX84 circuit I pinched it from but not in my SE5. It sounds spot on - I just need to nail this oscillation.

[pdf64]

The cascode may be working fine but all the extra stuff for the switching will be adding coupling to other parts of the circuit.
Consider trying wiring straight into and out of the cascode to eliminate that possibility.
Also it would be beneficial to narrow down which stages the loop is occurring around, eg does the master volume affect it?
Pete

[Willholmania]

The master volume doesn't affect it - just the gain control of the cascode stage itself. To switch it I've used one of those Weber channel switching board - little relay with a bridge rectifier and smoothing cap plus a diode across the coil. Runs off the heater circuit.

[Willholmania]

Actually, writing that last post has made me think that the buzz might be caused by interference from the relay's coil. I'm going to rewire the circuit so that the relay coil is off when the extra gain stage is in. At least I'll then know whether that's the cause of the interference.

[pdf64]

I can't see that the buzz would just kick in at a certain setting if it was interference? Rather it would probably come in gradually as the cascode output control was advanced.
Can your meter measure frequency, if so what is the frequency of the buzz? I assume that you've got a reservoir cap on the supply for the relay coil?

Does the stock gain control affect it? Or is the buzz solely affected / brought on or off by the setting of the cascode output control? Hope that makes sense.
Pete

[Willholmania]

My meter can't measure frequency, sadly. I think I might have to start hunting for an old 'scope. I suppose I could try some low value caps between the inputs of the stages and ground in car it is HF.