5f2a voltages

gdculley

@Modulus_Amps many thanks for coming back to me, and further reassurance my voltages are good. It's been a steep learning curve for me but I've really enjoyed building it and getting to understand more about how it works. I reckon I'll be buying another of your kits in the near future, I just need to make my mind up which one.....!

Modulus_Amps

gdculley said:

@Modulus_Amps many thanks for coming back to me, and further reassurance my voltages are good. It's been a steep learning curve for me but I've really enjoyed building it and getting to understand more about how it works. I reckon I'll be buying another of your kits in the near future, I just need to make my mind up which one.....!

The champ and tweed princeton are great starting points. while tinkering with the amp I would suggest you make sure you understand how to calculate the bias current/plate dissipation of the 6V6. If you are not clear I can outline it here for you.

When swapping rectifiers take note on the voltages and what you think the change did.
It is probably also worth trying different feedback resistors or put in a switch to break the feedback circuit and see how it affects the sound.

Keeping a notebook of all your findings may help you if you progress into other amps or even your own builds.

gdculley

Thanks. If you wouldn't mind explaining the method to check the 6v6 bias please, that would be great! I tried to follow the method on Rob Robinette's website but got a little lost, partly because I think it's specific to a 5e3, or possibly because I was out of my depth! 

Yeah I was planning on a negative feedback switch as my next mod, it seems to be a popular mod on these amps.

Thanks again!

ICBM

Measure the voltage across the cathode resistor. Divide the voltage by the resistor value, which gives you the current. Measure the cathode to plate voltage, and multiply that by the current, which gives you the valve dissipation. Calculated this way it does include the screen dissipation as well as the plate dissipation, but it’s small and the error is in the right (safer) direction so it doesn’t matter.

As always, be very sure you know what you’re doing since you’re working with voltages in the 300-400V range. It’s best to clip the probes to the terminals if you can, to avoid the risk of anything slipping.

gdculley

Fantastic! Thanks very much for your help. I'll try and get round to this over the weekend. I really appreciate you all taking the time to help with this stuff!

Dave_Mc

Excellent, thanks @springhead and @ICBM , I've ordered the rectifier valve and will report back on how it sounds when it gets here @gdculley (He had two of them, at least last time I checked, if you want the other one )

Wow your amp looks great @gdculley , I'm really impressed by anyone who can do this stuff, I can just about swap pickups in a guitar (and pretty shoddily at that!). That's interesting about the power switch- so it's more or less like an attenuator then? That's good news that it sounds really good, I'm not surprised the 25% power mod isn't enough for most amps. He seemed to be talking about amps around 15-20 watts, so if that only drops those to around 5 watts, that's still going to be pretty loud. And even a 5 watter down to just over a watt would still be fairly loud with even a vaguely efficient speaker.

ICBM said:

Dave_Mc said:

I did try a 6L6 in my VHT Special 6 and it was a lot louder and bigger-sounding than the JJ 6V6!

Remember that’s there’s an impedance difference - if the amp is properly optimised for a 6V6, using a 6L6 will actually *reduce* the power, and probably give a more middy tone (since it’s then a high mismatch). So I expect that amp has been purposely designed to take both types, or even optimised for the 6L6, which will then give more power and a wider frequency response.

Yep good point, you've mentioned that before of course, and of course I forgot! I suspect it is deliberately designed to take the 6L6 (and other bigger tubes too), IIRC it was designed to be easy to mod if you're that way inclined, and also to be able to use a lot of different power tubes. Usual caveat of "I couldn't compare the two valves immediately", as I had to leave time for it to cool and to get the back off and on again (I suppose I could've left the back off to save time, but then I figured that would change the tone too), but IIRC it seemed like a pretty noticeable difference- I'd be surprised if I imagined it, kind of thing. But I could be wrong! 

gdculley

@Dave_Mc thanks! I'm quite proud of it, especially the pine cabinet. Most of the rest of the credit goes to Modulus for supplying a very simple and clear layout for the build. Yes it is basically an attenuator I've added, it's definitely a worthwhile mod I feel. I had previously tried a tone king ironman 2 attenuator which is a great unit and goes quieter than this nod but I felt at the highest attenuation settings you lost some of the sparkle of the amp. I think this mod is a good middle ground and certainly a lot cheaper!

I've taken your suggestion and ordered the same valve as you! Many thanks for that, I'll report back on how it sounds...

Dave_Mc

gdculley said:

@Dave_Mc thanks! I'm quite proud of it, especially the pine cabinet. Most of the rest of the credit goes to Modulus for supplying a very simple and clear layout for the build. Yes it is basically an attenuator I've added, it's definitely a worthwhile mod I feel. I had previously tried a tone king ironman 2 attenuator which is a great unit and goes quieter than this nod but I felt at the highest attenuation settings you lost some of the sparkle of the amp. I think this mod is a good middle ground and certainly a lot cheaper!

I've taken your suggestion and ordered the same valve as you! Many thanks for that, I'll report back on how it sounds...

You're quite right to be proud of it! 

I've only tried the cheapo Harley Benton attenuator with mine, it definitely cuts a bit of treble. It does that with pretty much every amp I've tried it with, though, if anything it might not have been as bad with the Princeton! But it still didn't sound as good as without it... to be fair, some of that is the speaker breakup too I think, which by definition you're losing out on if you're not driving it so hard.

LOL nice score on the valve- I'll report back on mine when I've tried it too!

Dave_Mc

So I've tried the Philips 5Y3 and it's great- good call as usual @ICBM

I might be imagining it (but I don't think so) but I'm pretty sure it sounds noticeably better now- sweeter, more refined, I actually think it has noticeably less bass (that's more of a subjective thing as to whether it's an improvement, I'd say) than it did. Basically it sounds a lot better (and it sounded excellent already!). I'd say it was a similar level of improvement to the tone as changing to the Brimar 6V6 did.

I also think there's less hum and noise, too, and it even seemed to warm up faster! It seems "better behaved", for want of a better word. Plus sounds noticeably better too. For less than a new production one costs.

Awesome. 

gdculley

Well I got the same 5y3 as you @Dave_Mc and I agree I think it's better. It not a massive change but it does seem a little sweeter. I need to measure the voltages again now but have been distracted by patio laying and other boring things! I have managed to get round to adding a switch to remove the negative feedback circuit and it's absolutely fantastic! Such a simple mod and well worth it just to get a little bit more grit. I added the switch to the top panel so it's nice and easy to switch in and out.

Dave_Mc

@gdculley Mine seemed to be a noticeable improvement- maybe the JJ that was already in there was on the way out?

That's good news about the NFB switch- I've tried amps both with and without, but never the same amp with it on a switch...