Epiphone Valve Junior project

[TubeGeek]

One more thing...with the Hamptone's I find it easy to really set the gain properly.  I can hear the characteristics of the microphone in every detail. If I set up the microphone properly and dial in the tone of the amplifier then adding the 57 and preamp...I find it hard to get a bad recording.  They tend to add to the signal instead of taking anything away. Similar to vintage Neve pre's but modern :o

For what you pay for these kits I think you are actually getting much more than what you pay for.  They rank up with other preamps that cost 5X what these do.

I see a couple of little improvements that I could do on my hamptone's...thanks for motivating me to look at them again!


BTW Bassmanster...you have a great looking site too. I love my mac, it makes that stuff so simple to do.  I need to find a .com domain name and hosting site...who do you suggest?  I'd like to use iweb and publish to a folder and upload it to a host site.

[Bassmanster]

Thanks man!

I am using a friend of mine's hosting so he gives it to me for cheap, but if you wanna look it's out of barrierislandgraphics.com .

I have another site under imhosted.com.  Not bad for cheap.  It's easy to do stuff from the linux hosting control panel.  Well simple stuff which is all I do.

I publish to a folder and upload using CyberDuck.  At least one thing that I know of that won't work publishing to a folder:  the hit meter.  Otherwise I am happy.

Domain name:  I use NetworkSolutions but I have done so for a long time so I am used to them.  I suspect there are better bargains to be had.  I register the name myself so I have separate control of it instead of through my host.

[TubeGeek]

Regarding installation of a gibson ga-5 circuit:


I would like to use the power transformer already in the valve jr.  It has 6.3 VAC 260 VAC.  

First question:  With the schedule 40 ga-5 circuit...it has a 330 V PT secondary.  Can I use the 260V secondary safely?  
The 5V4 is rated to 500V 175mA.

Second question:  I have a 9 Henry 120mA inductor lying around...can I use this in replacement of a Hoffman 125C3A?

Third question:  What is the best way to get 5V from the PT...it looks like the stock valve jr has a 12V secondary...can I just rectify that down to 5V or is it best to use a seperate transformer?

Where can I find an output transformer for this amp that will have a 4,8,16 outputs?

Or should I just use another transformer? ::)

[PRR]

> it looks like the stock valve jr has a 12V secondary

Looks like 6V to me. Powers the EL84 directly. When rectified onto a big cap. we get ~5.9VDC.

But you can't power most 5V rectifiers from that winding, if it is feeding your 12AX7 and EL84 tube heaters. The filament of most 5V rectifiers sits at the B+ voltage, ~+360V; the heaters of the 12AX7 and EL84 have to sit near their cathode voltages, essentially zero volts, or within about 100V of that.

You can try to find a heater-cathode rectifier with 400V of heater insulation and maybe 40mA current capacity. 6X4 will work (except the Epi's HV winding is wrong for a common-cathode rectifier; see below).  

But I really think: you add another big bottle, you need to add heater power supply. If you are not going to replace the original main transformer, you will need an extra trannie just for the wrectifier. 5V at whatever current is ideal. A 6V winding may be easier to source: use Ohm's Law to figure a resistor to drop 1.3V at the rectifier's rated heater current.

Note that the 5V4's heater alone eats more power than the 12AX7 and EL84 heaters combined.

And you KNOW, for ~~$99, Epi didn't build any excess power capacity into that amplifier.

> output transformer for this amp that will have a 4,8,16 outputs?

The Head version has 4 8 16 taps.

> Can I use the 260V secondary safely?

That is a single winding, suitable for a Bridge (4 diode) rectifier. The 5V4, and most bottled rectifiers, are 2-diode with common cathode. What they really want is a center-tapped winding. Using the Jr's single winding, the only pure-bottle way to go is a Half Wave rectifier. Half of a 5V4 is plenty big for this ampifier, and the ~320VDC is fine, but the buzz will be bad and the power transformer will be unhappy about the half-wave load.

[radar01]

Hey TG, first just build the GA-5 around the Solid State rectifier and the existing power supply design already in the amplifer. The 2 12AX7 stages have major differences that make a big difference in the amps sound/gain.  I'm not sure where to tie the feedback if you decide to do so.

Alan

[TubeGeek]

That gives me some stuff to think over!

Thanks.

[PRR]

> The 5V4

Why use a bottle rectifier at all?

In push-pull class AB, the power stage sucks more current as output power rises. This causes the rail to sag. This "should" be "bad", but it seems to be good in hard-worked guitar amps. So a high-sag bottle rectifier gives a different sound than a fatter bottle or a sand-state rectifier.

However, the current drain of a single-ended stage does not change up to full "undistorted" power. When it does go into overload and gross distortion, supply current can increase, decrease, or even both depending on the signal and how bad it is mashed. But it really changes very little compared to classic push-pull amps.

So: does the bottle rectifier actually add anything to the single-ended amp? Or anything we couldn't instead get by fooling with bias and load? I haven't smacked the stuffing out of an SE amp in a very long time. As I recall, a low-feedback SE amp can give a wide range of tones with hardly a wobble in its current.

Gibson had to use a bottle: no choice back in the day.

The Sch40 uses a bottle because it uses a power transformer designed for a bottle: with a sand-state rectifier the B+ would be awful high. Anyway the Sch40 is as much a visual statement as a musical tool, and more bottles is more kewl.

Within the confines of a 99-buck amp's case, I'm not sure what a bottle rectifier offers except more heat (and slightly lower max power output).

> 12AX7 stages have major differences that make a big difference

That's my guess too. The Jr's preamp is functional but may be uninspired. Try trashing the silly PCB and using Classic Amp values and details in the preamp. Let the output stage alone for now.

Oh: do try mis-loading the output. My guess is that it will be more soulful with a "too low" load impedance: 4 or even 2 ohms on the 8 ohm tap. Unlike transistors, you can't hurt this class of amp with "overloading". Power output will drop a bit, but so?

[TubeGeek]

I just ordered my parts from Doug to build the ga-5 with a bridge rectifier instead of a bottle rectifier.

I bought the 3 amp/1000Piv rating, probably overkill but oh well.  

I am going to use a 9H, 120mA, 125ohm inductor.

I will drill holes and mount the turrets this week and hopefully build the amp early next week.  As soon as my parts arrive.

PRR...I ordered some 5k 5W resistors so I can try the switchable outputs you suggest

[Bassmanster]

I forgot how to make an archive request... :-[

Can this thread go in?  Lots of info on SE design.

[TubeGeek]

Hey bassmanster...speaking of the SM57....the latest issue of recording magazine has an interesting mod for it.  It is basically adding a resistor between pins 2 and 3 of the xlr in order to lower its impedance and match to most preamps better.  The article said that it changes the sound of the mic, better clarity. I thought this may interest you.  I will probably try this out and match it to my preamp input Z.  Another fun weekend project. ;D

[Bassmanster]

Thanks for the heads up.  I see it's not online so I'll have to find a real mag stand in this two horse county.

[TubeGeek]

I have a couple other mods for 57's also.  One involves removing the transformer.  If you cannot find the magazine, email me and I'll hook ya up!

[TubeGeek]

GA-5 install completed. Check my website for details.  

[GroundhogKen]

Thread archived.

I hope everyone was done contributing.


Ken

[TubeGeek]

I wasn't satisfied with the B+ being around 350V so I added two 22uF caps with a 1k 10 Watt resistor in between those caps in the epi combo I have...decreased hum to almost zero and brought B+ to 305V....which I can live with.

Now I am really done playing with this amp!
 ::) ;D

[Geezer]

Hey guys........your "new" replies have been moved to (click>>) [link=http://76.162.4.197/Forum/yabb2/nph-YaBB.pl?num=1172398164]This Thread[/link] (<<click) so that more people will see & be able to comment & help.
Geez'r