danelectro amp 6au6 preamp question

[SliperySlidey]

Hello Everyone,
Hello Everyone,

first post here.... I was wondering if anyone could tell me what the difference between a pentode and a triode is as it relates to the preamp. I recently got a Airline (danelectro) 8513 amplifier and it has 2 6au6 tubes in the preamp. The amps tube complement is as follows
1 6x5 rectifier, 2 6v6 power tubes, 2 6au6 preamp tubes. It has an old 12" Alnico Rola speaker which measures out at 3.2 ohms, the magnet
is so small that a washer barely will stick to this thing... but it sounds great! I am a lapsteel player and my steel just sounds great through this little gem.... The reason for the post is that I have never owned an amp with pentode tubes in the preamp....

I have attached the schematic...

here is a link to some pics of the amp
Airline 62-8513 Danelectro Corp., Red Bank, N.J. for Montgomery Ward and Co., Chicago, Ill.,USA

Also, if anyone can see any interesting things on the schematic I should know about... all input would be helpful

thanks everyone,

Smitty

[Gingertube]

A good simple design.
The 1st 6AU6 (Left Hand Side of schematic) provides high gain (higher than a triode) clean amplification.
The 2nd 6AU6 (in the middle of the schematic) is strapped as a triode and is a concertina phase splitter for the push oull output tubes.

All of the effective gain (main sonic infleuence) is in that 1st 6AU6 as the concertina splitter does not impart any gain.

A novel way of providing the screen voltage for that 1st 6AU6.

The Amp should sound VERY clean which suits lapsteel.

Cheers,
Ian

[SliperySlidey]

A good simple design.
The 1st 6AU6 (Left Hand Side of schematic) provides high gain (higher than a triode) clean amplification.
The 2nd 6AU6 (in the middle of the schematic) is strapped as a triode and is a concertina phase splitter for the push oull output tubes.

All of the effective gain (main sonic infleuence) is in that 1st 6AU6 as the concertina splitter does not impart any gain.

A novel way of providing the screen voltage for that 1st 6AU6.

The Amp should sound VERY clean which suits lapsteel.

Cheers,
Ian

Thanks very much Ian, It is a fairly clean amp, but the dynamics are amazing... depending on how hard I hit the strings I can go from
clean to a nice "crunch".... not sure if thats tube distortion or the ineficiency of the speaker... regardless, I am currently building a
solid pine cabinet for it, and will upgrade the speaker to a Weber 12a125a Alnico... should sound sweet.

take care,

smitty

[trobbins]

The 2.2M and 0.1uF is low enough corner to not introduce negative feedback between the stages. The grid leak bias on the first stage may be worth checking that the anode is idle at a reasonable voltage, and also a suitable anode swing range when a signal comes in and causes a bias shift - although probing the level with anything but a 10M input DVM would give a misleading reading.

What vintage is the amp?

Ciao, Tim

[SliperySlidey]

The 2.2M and 0.1uF is low enough corner to not introduce negative feedback between the stages. The grid leak bias on the first stage may be worth checking that the anode is idle at a reasonable voltage, and also a suitable anode swing range when a signal comes in and causes a bias shift - although probing the level with anything but a 10M input DVM would give a misleading reading.

What vintage is the amp?

Ciao, Tim

Hell Tim... thanks for the response... where guessing this is a 1957, simply by the speaker date, at some point I will check pot dates .... but
I think its safe to assume a late 50's something... Unfortunately my engineering skills are a bit sub par when it comes to this stuff, I'm just
a tinkerer, but am learning all the time.... so some of the stuff you were talking about in your post was above my head.... were you basically
talking about checking where the bias is set at? Thats something I want to learn how to check out for sure... thats part of why I joined
this thread, which is to try and start doing some of the work myself....

thanks again

[trobbins]

I think it's fair to say that grid leak bias was only used to any extent up to the 50's, due to expense, or lack of electrolytics for cathode bypassing, and the aim of using one less resistor in the circuit. Unfortunately it appears to be fairly unreliable (in the sense of accurate bias setting) and high value resistors of that era are likely to have driffted substantially. But probing the grid to sense voltage will load the circuit, so that can't be used to check performance. I think the best technique for checking bias setting is to look for clipping of the signal passing through the stage. Possibly a quick check of the anodfe voltage would indicate of bias was way off the mark - but it loads the circuit as well, and the bias level changes with signal as well. Still - it's fun, and well worth keeping as is.

[RWood]

A novel way of providing the screen voltage for that 1st 6AU6.

I don't see how the screen is getting any dc voltage. Care to elaborate?


RWood

[trobbins]

From the lower arm of the phase inverter - which is RC filtered to remove any AC signal that is on the phase inverter lower arm.

[tubeswell]

here is a link to some pics of the amp
Airline 62-8513 Danelectro Corp., Red Bank, N.J. for Montgomery Ward and Co., Chicago, Ill.,USA

Smitty

Hi Smitty, and welcome to the forum

That sure is one elaborate looking baffle mounting set-up. I'm intrigued by how they get a good sound out of the 6AU6s. I have a pile of those tubes lying around so I guess this goes onto my rainy-day project list.