Uses for these tubes??

[Govmnt_Lacky]

I put out a few feelers to some of the "tube savvy" forumites and have not received any responses so, I will put out an ALL CALL.

I have the following tubes that look interesting and may be well-served for a stompbox or amp circuit. I am not experienced enough in tube territory so I figured I would ask for info 

I have the following:

6LE8
6GH8A
3HA5

Any ideas on these would be great 

Thanks in advance!

[merlinb]

6LE8 is a funny pentode with two anodes and screen grids. They could be hooked in parallel to make a single 4W pentode for a small SE power amp.

6GH8A is just a triode-pentode and could serve as the 1st and 2nd stages of a preamp, e.g., original Vox AC30 style. Alternatively, wire the pentode as a triode and make a Valvecaster? Or even leave it as a pentode and make a Valvecaster mutant...

3HA5 is a triode. Does what it says on the tin really!

[tubelectron]

Hi Govmnt_Lacky,

The 6GH8A is used as a gain stage (pentode) + phase splitter (triode) in the Scott 222C Hi-Fi amp from early 60's, like the one I own. These are the 2 tubes around the center filter cap :




the 6GH8A is similar to 6AN8, 7199 (as used by Dynaco for the same role, in the same era)

A+!

[Govmnt_Lacky]

Thanks Bruno and Merlin 

I am really noobish when it comes to tube design and building. I guess it will just take some breadboarding and time.

Now... I just need to know where to start! 

[bluesman1218]

I am really noobish when it comes to tube design and building. I guess it will just take some breadboarding and time.

Now... I just need to know where to start! 

Just remember one word in the realm of tubes... VOLTAGE. Although I sometimes wonder why tubes are used in low voltage apps when, by their very nature, they offer the 'best' tone when pushed. Just thinking through the keyboard!

[tubelectron]

Yes, bluesman1218,

Just remember one word in the realm of tubes... VOLTAGE. Although I sometimes wonder why tubes are used in low voltage apps when, by their very nature, they offer the 'best' tone when pushed. Just thinking through the keyboard!

I agree completely - even if I never took the time to test anything seriously with tubes at low voltage ! All of the following stompboxes are high voltage, the Westbury W-20 being probably one the first commercial offer (1978-1982), the others being from me. The W-20 is not well renowned, but it is nonetheless a simple device and a very good sounding unit...



A+!

[tubelectron]

Govmnt_Lacky,

Now... I just need to know where to start!

Here is the schematic of the W-20, thanks to R.G. Keen :



It uses a 12AX7, but your 6GH8A would do, with some mods : 1st stage in pentode, 2nd in triode, for example... Place a 220 to 470K serial + 0,1 to 0,47µF to feed the screen from the +HV, as a start.

A+!

[bluesman1218]

Very nice pedals, Bruno. I have a 2 - 12AX7 preamp pedal I need to finish (Doug Hoffman's design).

I'll use it for the few times I play electric through my solo rig (Fishman SA-220) and just add a pedal or two.

[amptramp]

The 6LE8 was originally designed as a high-level synchronous demodulator for colour TV.  You could make a double-sideband modulator out of it that could give you sum and difference frequencies for two incoming signals.  You could make a ring modulator with isolation and gain.

The 6GH8A is probably the most common tube from a 1970's TV.  You can make a good pentode first audio / triode split-phase inverter to feed a push-pull audio output stage in an amp.  It is an improvement over the 6U8 and shares the same pinout.  In fact I might replace the 6U8's in my H.H.Scott 340B FM receiver with 6GH8A's - it is a good upgrade.

The 3HA5 is a small (the size of a 6AL5) RF amplifier triode that features low noise and low output impedance.  I just bought two of them because they offer low noise at moderate plate voltages - they are rated for 135 volts on the plate.  The high transconductance makes it a low-noise device and the Rp of 4965 ohms means it can tolerate low plate resistors for better bandwidth without reducing the stage gain too much.  Transconductance = Gm = 14500 micromhos and µ = 72.

[bluesman1218]

If you need Tube Data Sheets, this will help. http://tdsl.duncanamps.com/tubesearch.php

Click on <Tube Data) on the left and take it from there. It will give you basic data and links to PDF data sheets you can download.

[Govmnt_Lacky]

Thanks for all of the replys 

I have been thinking about it and I may just offer up these tubes for sale. Just don't have the amount of time to invest in a full blown tube project right now.

Hopefully someone will want them 

[Paul Marossy]

The only thing I think with these "oddball" tubes is if I build something with them, how easy (and expensive) is it to get replacements?

[PRR]

> I am really noobish when it comes to tube design and building.

Then go with Known-Good Well-Documented plans using affordable tubes.

Using a tube just because you have it is The Hard Way. 12AX7, EL84 are NOT expensive to buy and are NOT the major cost of a tube project.

I built a super-Champ recently. The base set of tubes (one spare output) cost $65. The total build with chassis, knobs, transformers, tagboard was over $500 (it was a bit lavish).

Later, when you are "expert". you can "make something from anything". My contractor is making a window out of a 6x12" laminated wood beam... yowsa.


> 3HA5 ... low plate resistors for better bandwidth without reducing the stage gain too much.  Transconductance = Gm = 14500 micromhos and µ = 72.

Well, uh..... you MUST reduce plate resistors. it works great at 8mA or 16mA, less-well at lower currents like the 1mA typical of non-extreme audio stages. 6AH5 is designed to work less-well at low current to provide AGC in radio systems. 

A typical audio stage might run 1mA and 135V (100K load to 240V B+). At this point, Gm ~~ 1,000 micromhos (no better than 12AX7) and µ ~~ 35. Voltage gain will be near 25-30. Zout near 25K, lower than 12AX7, and Cgp aint bad.

As Ron says, 6LE8 is a very special-purpose bottle. As Merlin says, you could develop a power amp from it. The high heater demand, low plate rating, and soft knees make a EL84 look like a better engineering choice, and EL84 amp plans are everywhere.

6GH8 is a fine audio tube. 1/5th of a great hi-fi amp. Yeah, you could adapt some smaller plan to use it, but that could be daunting for a beginner.


> just offer up these tubes for sale.
> I think with these "oddball" tubes is if I build something with them, how easy (and expensive) is it to get replacements?

6LE8 is a buyer's market. They were plentiful and there's about zero demand. 6HA5 "could" become faddish if someone posts a puffy page about how wonderful it... even then, the odd-brothers like 3HA5 tend to be shunned (it took years after 6SN7 got faddish for 12SN7 prices to rise).

Socket the top of a pedal-box, put LEDs in the sockets, and CALL it "a tube pedal" even tho the tubes do nothing. (And if 3HA5 prices do ever rise, you'll have one.)