Greta v1 and v2 layout.

[Linomark]

Hi I am new to this forum. I recently picked up a 72 RI Tele deluxe and a Greta amp. Im really digging the tube tones that I get from this setup. I tore an old 50s multimeter apart that had tubes in it and it had two nice Sylvania tubes a gold brand 5751 and a 5814A green label. I want to try these in the greta but Im confused about the tube orientation. From what Ive read up on concerning tube amps the V1 socket is almost always the preamp position this would hold the 12ax7 5751 or 5814a which is JAN 12au7 equivalent. My problem is I bought the amp used and am not if the tubes are in correctly. The label on the bottom of the amp lists the 12at7 tube in the v1 position to the rear of amp and the 12ax7 as going in the v2 socket. This seems backwards from everything I have read on Fender amps. Can anyone tell me which socket is the preamp and which is the power. Great forum and loving my Tele and Greta. Thanks Mark.

 

[Glen Smith]

The 12 AT7 is V1. However the schematic I found doesn't show them the way we usually see them. They show V2 as preamp and V1 as power.
http://www.tonebuster.org/home/images/amp_greta/fender_greta_schematic.png

 

[jimash]

Yup its backwards
I will leave it to others to decide which one could go where.
http://www.tonebuster.org/home/images/amp_greta/fender_greta_schematic.png

Damn

 

[Linomark]

Greta

I think I have a bad power tube. When I run the power tube in the v1 position as the label on the bottom of the amp suggests the amp crackles loudly when playing. I have tried the ax7 in the power slot and a 5751 in the preamp slot and the amp plays fine much lower gain though. I tried 5751 in the power section and a 5814a in the pre amp section and the amp plays fine just very low gain. I reversed this combination and the amp played fine a bit more gain this way. Im stumped. I know its okay to run either of these tubes in the pre amp section. I want to know if its ok to run either a 12ax7 5814a which is an au7 or 5751 in the power section

 

[jimash]

Generally, and based on my experiences with the Hughes and Kettner Cream Machine, you could run the au7 in the power slot.

 

[Silverface]

The 12AU7 is fine in the power slot. It draws less current than the stock 12AT7 power tube and is lower gain.

The preamp tube (stock) is the 12AX7 and the power tube the 12AT7. This is the first time I'd seen the schematic (I'm happy I have it, since I have the amp...now I can start screwing it up! ;-) - the "V" numbers are screwy on the schematic (which was done in German!), so just ignore them (they also used "-V,--V etc where we in the US normally note letters of the alphabet ("A", "B", etc) also found in the power supply, for "B+" voltage).

You can virtually run any of these (or the number equivalents - 7025, 5814, 5965 etc) tubes in either socket:
12AU7, 12AV7, 12AT7, 5751 or 12AX7. You might even try a 12BH7 if you stumble on one; they were used in many Ampegs and are essentially 1/2 12AX7 and 1/2 12AU7. It would be a lousy driver tube where a balanced tube is required, like in a BF/SF long-tail driver/phase inverter circuit, but in the Greta each tube section is being used as an individual triode (all the listed ones...and required for this amp...are dual triodes).

You don't see 12AV7's mentioned often as they draw more current than a 12AX7, are low gain and often microphonic...but if you get a good one in a circuit (like this) that could handle it in the power slot it might sound pretty darned good.

I think I have a bad power tube. When I run the power tube in the v1 position as the label on the bottom of the amp suggests the amp crackles loudly when playing.

Not necessarily. Might be just dirty pins (in fact, that's exactly what it sounds like. I'd use a couple twists with a piece of 1000 grit sandpaper wrapped around each pin, followed by dabbing each one with any one of the Caig cleaning products (lubricant is not necessary on pins - it's VITAL in pots!). Let it dry before reinserting and see if the crackle goes away. If not, it may be a bad tube (but I wouldn't toss it - give it to someone who has a tester at least!).

 

[Wagster]

You don't see 12AV7's mentioned often as they draw more current than a 12AX7, are low gain and often microphonic...but if you get a good one in a circuit (like this) that could handle it in the power slot it might sound pretty darned good.

I'm glad you mentioned this about the 12AV7. I ran across a couple a few years ago at a "you have no need for them but to low to pass up" price. I did try them in a few different slots in my SF Twin but they were pretty bland.

If I'm ever again motivated enough to remove all screws in that dang cover, I'll give one a test drive in the Greta.

Hmmm....might even stick one in my Marshall Class 5 just for fun......

 

[marshman]

The 12AV7, like the AU and AT, were not designed for audio, and typically sound a little iffy when used as such, but sometimes, they just sound right. I was tube-rolling my Valve Jr a few years back and the second-best sounding preamp tube I tried was a used, vintage TungSol 12AU7...go figger. Given the choice of a 12AU7 and a 12AX7, I'd say use the AU as a 'power tube'.

I think I read that the AV7 was designed for use in early computers, but I can't prove it.

 

[DaveKS]

Generally, and based on my experiences with the Hughes and Kettner Cream Machine, you could run the au7 in the power slot.

Yep it should drop right in the 12at7 slot, but will drop vol a bit and maybe just smooth it out.

That 5751 is one best tubes around, replace that 12ax7 with it.

 

[Linomark]

Thanks for all the positive feedback. I used the Sylvania green label 5814-A in slot closest to the back of the amp, I believe that is the power section. The Sylvania gold label 5751 went in the socket towards the front of the amp the pre amp section. This combination completely changed the tonal output of this little amp. For some reason the amp has much more clean head room. Its hard to describe but the tones are bell like, singing sustain and so forth. The break up is more gradual as well. Whats odd is that the VU meter will peg all the way into the red when cranked now and the amp is still pretty loud. The tubes seem to run pretty hot when turned up as well. I checked the transformers and they get warm after a session but not hot. Im loving the tone and dynamics of this amp alot. I just hope the tubes last awhile.

 

[DaveKS]

I'd keep the 5751 for sure. I'd probably hit eBay and try to pick up some cheap real nos mullard cv4024 (12at7) and sylvania 6072 (12ay7) to try instead of 5814 in power amp slot.

6072 in preamp and cv4024 in power amp would also be awesome too.

 

[DaveKS]

Hey Wag, yea 12av7 left me wanting too. Sylvania 6072 will rock your world.

I'm glad you mentioned this about the 12AV7. I ran across a couple a few years ago at a "you have no need for them but to low to pass up" price. I did try them in a few different slots in my SF Twin but they were pretty bland.

If I'm ever again motivated enough to remove all screws in that dang cover, I'll give one a test drive in the Greta.

Hmmm....might even stick one in my Marshall Class 5 just for fun......