New poster needs help and advice

[sprog]

Hi all,

I have built a few hi-fi valve amps and have been asked by a friend to assist with stripping down his 40 year old Miazzi amp and build something better. He says he has never liked the sound of it. I have salvaged the transformers and 4 X EL84, 4 X ECC83 and 2 X EZ81

I intend using ss rectification but need know if this would detract from the sound in any way and if OK then direction to a ss PSU schematic would be helpful. He plays Beatles to blues to rock. The output transformer is correct for PPP so I have been looking at the 36 watt designs. Not being a guitar player perhaps someone can suggest a schematic. I would like it to be as simple as possible with only one input but retain bass, mid and treble control. He also wants to be able to drive into distortion at low volume when practicing at home

Regards

[snoof]

As far as the SS power supply goes, it will depend on what kind of PT you are using. Is it center tapped?? As far as the tonal changes from switching from tube rectifier to SS, there will be less sag in the PS (less compressed feel to the amp when running hard) and the voltages will be be a bit higher than w/ the tube rectifier.

If you are looking for a one channel TMB based design, I'd recommend the Superlite. Just double up on the power tubes...

http://www.18watt.com/files/18wattSuperliteTMB.gif

Although you might possibly run into a problem with your heater voltage being a touch high because of only using 2 of the 4 preamp bottles that your PT is speced for.

Of course you could also go with the standard 18watt TMB design, and leave out the "normal" channel, and double up the power tubes...

http://www.18watt.com/files/18wTMB_Richie.pdf

[CurtissRobin]

Although you might possibly run into a problem with your heater voltage being a touch high because of only using 2 of the 4 preamp bottles that your PT is speced for.

???? A heater voltage rise might occur if the PT's heater winding was overloaded in the original Miazza design but that's unlikely.

sprog, those transformers and tubes should work fine in a 36 watt amp and with the SS rectifier it'll be stiff and the B+ voltage higher than with a tube recto but you could include a sag resistor in the B+ line for more tube recto-like behavior. There are also a lot of threads here about using zener diodes to reduce B+. Here's a link to our 36 watt download page http://www.18watt.com/modules.php?name= ... oad&cid=45

HTH
KennyO

[sprog]

Thanks for the response.

Hi snoof, yes the PT is center tapped. I measured the heater voltage at 6.1V before I stripped the amp. I will in fact be using 3 less valves than the original. 1 X ECC83 and 2 X EZ 81 which also share the heater supply, however I do not expect the lighter load to increase V past 6.6V. If its a problem I can Connect the PT to the 240V tap instead of 220V.

Hi CurtissRobin, what do you mean by a sag resistor, what value and where does it go.

Regarding the NFB, will the component values change bearing in mind that I will be using paralleled 12" 8 ohm speakers.

Can anyone using a SS PSU suggest suitable values

[snoof]

Since the PT is center tapped, then just use 2x1n4007's per side (full wave recto). I suggest getting Duncan's PS calculator to design your B+ rail.

http://www.duncanamps.com/psud2/index.html

There is no NFB in the common 18watt versions. The speakers will make no difference in NFB values, it's the tap that you pull the signal from that makes the difference which values you use(in your case 4 ohm tap).

I don't see how your heater voltage cannot rise, those 2xez81's are pulling 1A each. That's freeing up 2.3A of current all together(ez81's and 12ax7). Is there a separate heater wind for the ez81 heaters?

[zaphod_phil]

Since the PT is center tapped, then just use 2x1n4007's per side (full wave recto).

Or UF4007s may be even better in terms of noise.

A sag resistor is sometimes used after SS rectifiers to simulate the sag effect of a vacuum state rectifier. According to a recent discussion we had, it appears that the internal impedance of an EZ81 is approximately 130 ohms, so that's the value you would need to use if you wanted to simulate an EZ81.

[sprog]

SS power supply recommendations anyone ?

snoof suggested the 18watt Superlite TMB and this is what I will build but with 4 X EL84. Searching around for a ss PSU just has me confused. Some designs use a series of low value caps around 22uf and others use 470uf.

You may wonder why I don't just stay with the EZ81's for rectification. Firstly, to simplify things, for reliability and to save space on a new chassis, and secondly because I need these scarce valves for another project.

It is just the PSU that is holding us up now. I have played around with Duncan's simulator and though I don't fully understand it, I seem to be able to model a supply with low or high value caps, but this does not tell me what would be suitable for guitar.

So advice from experienced players would be welcome.

[snoof]

I say just go for it with whatever values you have. I personally don't like using the higher values (like above 47uf) cause the amps seems a bit stiff to me.

Where the Duncan prog shines is that it allows you to diddle the resistors in the rail to get the voltages that you need at each node. But you gotta have the right specs for your PT to input.