[funch]

FWIW, these guys Antique Electronic Supply have 6GM8's for
$5.40 each. I placed an order for 5 yesterday 'cuz that's all they had,
so I figured I'd snatch 'em up and pass the deal along to those who
were building an amp, but this evening their website shows that they
have 6 more. Dunno what's up with that. I'll post the brand when mine
come in.

 

[funch]

What a grab-bag lot of 5.
1. Amperex - made in Germany, original Amp. green and yellow box.
2. Amperex - made in Holland, ditto on the box.
3. Bugle Boy - made in Germany, in new white box.
4. GE - made in Holland, orig. box.
5. Admiral - made in Holland, Admiral 'Supertron Tube' box.

I'll try to post pix tomorrow. Now I see AES is out of them again.

 

[funch]

Pix:

Good news: 6GM8's for cheap.
Bad news: not a match in the bunch.

 

[digger945]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Bad news: not a match in the bunch.

They don't test the same?
Many tubes can be identical and have all sorts of brand names or print.

 

[funch]

Oh, I have no way of testing them. I was just saying that none of them are
the same by appearance, although they all may be Amperex manufacture.
I really don't know enough to say.

 

[johnwmclean]

Thanks funch, for letting us know!

 

[jjazzyj]

Quote:

Originally Posted by funch /img/forum/go_quote.gif Oh, I have no way of testing them. I was just saying that none of them are the same by appearance, although they all may be Amperex manufacture. I really don't know enough to say.

I always buy my tubes from AES. They have the best prices on any tubes. I ordered my 6gm8's from them 6 months ago when they were on sale for 4.80. Out of the 8 that I ordered 6 were amperex and 2 were Zenith made in Germany so in other words amperex made those too. Tube rebranding happened alot back then.

 

[johnwmclean]

The EHHA I’m building is for speaker and headphone use, I’ve become stuck with switching/wiring, would it be a simple matter of wiring up the outputs for speakers, is there a need for switching? I will be socketing all gain positions and a zobel network will be added to the speaker jacks. Looking for the best way to implement this into the build, I’m using customs heatsinks, which will allow me to keep the FET’s on board so oscillation shouldn’t be an issue.
Thanks guys!

 

[runeight]

Good question. Did we change the value of R13 for the speaker build?? If so, you might need to switch here for HP or speakers. If not you're good.

The zobel shouldn't affect the headphones too much, but if you really want to be pure about it you could switch those in/out too. However, you'll have to be careful about the leads to/from the zobel as the zobels need to be on the jacks if possible and leads will add some small amount of inductance.

This is not an area where I have looked deeply into best practices etc. so others may have better info or suggestions.

These changes would be enough.

 

[johnwmclean]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif Good question. Did we change the value of R13 for the speaker build?? If so, you might need to switch here for HP or speakers. If not you're good.

Alex, R13 is 1k2 which is the default value, under TWEAKS > Power Amplifier it remains unchanged.

Looks like I can omit switching

 

[sachu]

I gave the power amp tweak a try and it burned two pairs of my output mosfets immediately. Was too tired with the problems in my EHHA so ended up defaulting to the speaker setup for the VBE multiplier.

So are you going to go speaker out on the VBE multiplier John?

I do have a switch on my amp however..I have the amp wired to the binding posts with a zobel (but VBE multiplier is for headphone loads only)..I use the switch (an On-ON DPDT switch) so I can just use it when I turn on /off my amp or change headphones without having to turn the volume down.

 

[johnwmclean]

I’m very sketchy with this Sach, I was planning to use the same outputs for BOTH headphones and speakers.
I guess from what you’ve said they need to be kept separate, as they need different voltages? I’m also in lala land with how the VBE multiplier thingy works in this situation, so, from PCB, where would the I connect outputs for speakers?

 

[sachu]

No,

the same outputs go to the speaker binding posts as well..jsut that on the binding posts you have the zobel..

With a 10A DPDT (in you case you will need a 4PDT for balanced) on-on switch, the output of the amp boards go to the input of the switch and from there you will wire one possible output set of pins on the switch to the speaker (put the zobel in there on the binding posts) and then the other set of outputs from the switch go to the headphone out.

My amp boards are configured for headphone load, but I am guessing i should be able to run small efficient speakers with it as well.

The VBE multiplier controls the operting point of the output MOSFETs..in the power amp configuration with R20 being smaller (100R trimpot) you push though a higher voltage out to the gates of the mosfets, thus you need to replace the output resistors with smaller ones at 0.47 ohms instead of th standard 2.2R which is used for headphone loads.. If any of this is wrong, please feel free to correct me folks.

 

[nattonrice]

John, the zobel network should be (air) wired directly to the binding posts.
If you are worried about mechanical stability just you a small dab of hot glue to fix the cap and resistor to the back panel next to the post.

 

[johnwmclean]

sachu, that’s the info I need! Thanks a bunch for all the help you’ve been thus far through this thread.

Tom, that’s a great tip! Cheers.