GM70 PP

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simon
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Re: GM70 PP

Post by simon » Thu Oct 10, 2019 2:39 pm

The intrigue was getting the better of me so it's interesting to hear the background Nick. Yars ago I had money taken from my card and had to wait a couple of months for the stuff, but thats not unheard of in many businesses. But borrowing money is on a different level. I suspect a one man band selling components for valve amps is a lonely business, with cash flow problems, at times.

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Nick
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Re: GM70 PP

Post by Nick » Thu Oct 10, 2019 3:34 pm

I suspect a one man band selling components for valve amps is a lonely business, with cash flow problems, at times.
I am sure, and it wasn't the borrowing money that was the problem, its was the decision to not pay it back. After prompting by a third party a couple of years later, he rang again and listed a number of reason why he hadn't paid, and promised to do so ASAP, but it never happened. Not a case of forgetting or being short of cash for that many years.
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Re: GM70 PP

Post by simon » Thu Oct 10, 2019 4:56 pm

Indeed not. I can imagine there are deeper issues but I don't know so shouldn't comment. But I didn't mean to imply it was okay what has happened to you, far from it.

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pre65
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Re: GM70 PP

Post by pre65 » Thu Oct 10, 2019 8:37 pm

Nick wrote:
Thu Oct 10, 2019 3:34 pm


After prompting by a third party a couple of years later, he rang again and listed a number of reason why he hadn't paid, and promised to do so ASAP, but it never happened.
I had "helped" Philip Ramsey with several short term loans, and he always paid me back on time, and did me several very favourable deals as a reward.

But when I found out he owed Nick a substantial sum I told Philip I would not have any further dealings with him till Nick was reimbursed.
Life is what happens to you while you're busy making other plans. John Lennon

G-Popz THE easy listening connoisseur. (Philip)

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Greg
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Re: GM70 PP

Post by Greg » Fri Oct 11, 2019 12:14 am

If you want Hammond Transformers, you don’t need to go through a third party. You can buy direct from Hammond which I’ve done in the past.

https://www.hammfg.com/electronics/transformers

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Re: GM70 PP

Post by izzy wizzy » Fri Oct 11, 2019 8:15 am

Thanks for the info on Bluebell. I've come unstuck with loans to good friends who are now no more due to non payment. It stinks. I would find it hard to support someone who treats others this way.

Hopefully later today I'll try the 100H lundahls to see the effect of them in the bias.

Cheers
Stephen

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Re: GM70 PP

Post by izzy wizzy » Sat Oct 12, 2019 1:47 pm

Adding the Lundahl 100H choke, which is really an anode choke, to the bias, changed things for the better in a number of ways, especially tidying up the top end and once again, changing the presentation/aural matrix. This is now going in a direction I hadn't considered; that the bias supply would have such an impact on the sound.

I have had a certain "one note" issue in the bass with the ESLs and now the Tannoys for as long as I can remember. This aberation is now much diminished with the changes to the bias supply. I thought the issue was either the speakers, the room or both and have just lived with it. Is having had a PP valve amp in my system with a crude bias supply (the EAR509s and now this) been the issue all along?

All the research I've been doing on bias supplies focus on either the reasons why they can be so simple or the need/requirement to maintain balance in the output stage. The simple designs all seem to conclude, there is little to no current drawn, so crude rectification like half wave is all that's needed and 10s of millivolts of ripple is fine. It might be fine in that it won't hum but I think there is more to it than that now. The methods that focus on balance start of crude and seem to end in some kind of servo architecture which is not something I want.

That the bias supply has such an impact shouldn't be that surprising yet is not something I've seen much info on. Whatever noise is on the bias supply is being injected into the finals. Yeah PP cancels blah blah but that doesn't stop each valve from processing that ripple injection in a different way and then combining it again which I would imagine creates all kinds of intermod artifacts. Best not inject such a noise in the first place. It is this character I think I'm hearing now?

Nowhere else in the chain would I consider a cap input supply fed from silicon diodes and yet, that is what I have. Do I need to consider a valve rectified, choke input supply? Or do I look at some kind of adjustable self bias situation which I did have in some of the previous versions. Much to think about. All ideas welcome.

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Stephen

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Nick
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Re: GM70 PP

Post by Nick » Sat Oct 12, 2019 2:13 pm

If you are using a valve rectified bias just make sure its up to voltage before the HT.

To me the idaa that a crude bias supply is good enough, is like the idea that a ecc83 can drive a gm70 as its not going into A2 so there is no grid current. i.e. rubbish.

It always amuses me that a lot of folk seem happy to obsess over things like what make of resistor is used, but ignore the obvious like getting ALL the power supplies right. I guess its because the resistor is simple, the other takes more work
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Re: GM70 PP

Post by izzy wizzy » Sat Oct 12, 2019 2:32 pm

Nick wrote:
Sat Oct 12, 2019 2:13 pm
If you are using a valve rectified bias just make sure its up to voltage before the HT.

To me the idaa that a crude bias supply is good enough, is like the idea that a ecc83 can drive a gm70 as its not going into A2 so there is no grid current. i.e. rubbish.

It always amuses me that a lot of folk seem happy to obsess over things like what make of resistor is used, but ignore the obvious like getting ALL the power supplies right. I guess its because the resistor is simple, the other takes more work
I have a two switch start up coz of the MVs but yes, good to think of the start up in the bias too.

I'm all about the topology first over the obsession with what you're talking about. But I guess it can sometimes be obsessing with what you have control over i.e. mains cables, interconnects and the like vs the work on topology. So yes, I think we're on the same page.

Cheers,
Stephen

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Nick
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Re: GM70 PP

Post by Nick » Sat Oct 12, 2019 3:46 pm

But I guess it can sometimes be obsessing with what you have control over i.e. mains cables, interconnects and the like vs the work on topology.
Exactly, I think that’s why its potentially hard for DIY types like us here to understand the more mainstream HiFi world.
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Re: GM70 PP

Post by RhythMick » Sun Oct 20, 2019 6:56 pm

For me I think having the bias supply absolutely ripple free as far as possible is paramount. Irrelevant for me as it happens as I've switched to cathode bias, but when I had fixed bias I went for separate choke filtered PSU right back to separate tx secondary winding. I prioritised the smoothing of the bias supply over all others.

Re Nick's point, it's not just making sure bias comes up first, it's what to do if the bias supply fails (eg valve rectifier fails). It can get complicated to failsafe this but I ended up using a relay in the heater supply for the rectifier feeding the audio valve, coil driven from the bias supply. This worked well and I didn't notice any noise - I think I used SS relays (optocouplers).

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Re: GM70 PP

Post by RhythMick » Sun Oct 20, 2019 6:58 pm

What model Lundahl choke, where in the filter chain and how is it wired?

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Re: GM70 PP

Post by izzy wizzy » Tue Oct 22, 2019 1:01 pm

I started with cathode bias but with all the other issues going on in earlier builds and datasheets not recommending cathode bias (813) I ended up with fixed bias as simple as possible to get it going. Seperate Tx, bridge, 680u. Only then did I add Danbury 10H 20u then swapped that for 100H Lundahl LL1668, coils in series. PSUD says 150uV of ripple which is very low in my book.

However if before it had the sonic signature of a big electro, it now has the sonic signature of a my phono of about 15 years ago; that of a not very good metallised PP capacitor which was throughout the power supply; Ansar, which I don't particularly like. Sure most of it is good/better but there's something going on which get's on my nerves. One of those things, once heard, begins to annoy a bit. But this is a small thing overall. I struggle with met film caps.

But it makes me want to try cathode bias again as I really don't like tuning my amp by messing with capacitors. There is the heat and balance issue as well. Was not expecting to be in this position with the bias supply. It's an order of magnitude lower in impedance than most bias supplies so maybe that's why? Grid current? No bypass on the grid?

Cheers,
Stephen

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Re: GM70 PP

Post by izzy wizzy » Thu Oct 31, 2019 1:03 pm

So pleased with the bias improvements, going to leave that for a bit.

Got these built up ready to be tested and popped in when I get a chance

IMG_20191031_125905387_compress98.jpg

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Re: GM70 PP

Post by RhythMick » Thu Oct 31, 2019 1:10 pm

Slightly confused. They are the small boards but with the 3 legged transistors mounted on the wrong side? How are the heat sinks mounted?
Last edited by RhythMick on Thu Oct 31, 2019 3:00 pm, edited 2 times in total.

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