Akai 707 Preamp Modification

[untune]

If you are using the VTB2291 there is no connection from it to the HT. The 5K1 goes from the EL84 anode direct to the HT just like in the other stages. The 4.7uF goes from the anode to one side of the 2291 primary and the other primary connection goes to 0V.

Cheers

Ian

Thanks so much Ian, that makes much more sense. That's the last piece of the puzzle, pretty much - after further study I think the 25uf cathode bypass caps might be a bit low (in the 3rd stage at least) so I'm going to experiment with bigger values there but other than that, it's ready to assemble.

Cheers
Lee

 

[untune]

Morning all - build is slowly coming along, just a couple of minor things I'm curious about.

Firstly, with the triode strapped EL84 valve - I've seen some diagrams with a smallish (100 ohm) resistor between the anode and screen. I assume the purpose is to tame parastic oscillation should it occur; I had a spare 100 ohm resistor from the old SE output tranny that I could reuse, but since they're the old ceramic/carbon film tube resistors, I've no idea what wattage they are or what is best in this instance. I'm guessing these are 1/2W at most - will it be too little considering it's between plate and screen?

Secondly grid stoppers - same again with the EL84 I've seen ~1K stoppers - will it be necessary, or ok to omit (at least temporarily?) I only have two 1K2 1W resistors to hand, are they strictly required for the other triodes too?

Cheers
Lee

 

[ruffrecords]

The EL84 has a relatively high gm so it can be prone to oscillation. I would say the 1K grid stopper is mandatory, There is no harm in including the 100 ohm between screen and plate. Wattage is not important because the resistance is low so the dissipation will be low.

Cheers

Ian

 

[untune]

The EL84 has a relatively high gm so it can be prone to oscillation. I would say the 1K grid stopper is mandatory, There is no harm in including the 100 ohm between screen and plate. Wattage is not important because the resistance is low so the dissipation will be low.

Cheers

Ian

Thanks Ian! I've left it in place and also used a 1/2 watt 1k that was originally a cathode bypass resistor as the EL84 grid stopper.

Made good progress and can probably get it into a state to test later tonight; I've been having thoughts about the volume pot, and that it might be more useful between the second 6AU6 and the EL84 as opposed to between each 6AU6. Any thoughts, or worth trying anyway? I tested it yesterday and despite being marked as 500K it acutally appears to be closer to a 1meg pot.

PS I received the valves you posted at the weekend, many thanks

 

[ruffrecords]

You have 20dB gain from the input transformer and about 30dB from the first stage. If you do not put the gain control just after the first stage there is a danger of clipping the second stage. Also, if you put it after the first stage, you leave open the option to apply NFB from the EL84 back to the second 6AU6.

Cheers

ian

 

[untune]

Thankyou Ian, I hadn't considered that

Well tonight I got this to a point where I could test it (left the mic input/transformer off for the time being) with the intention of just making sure I could get a line level through it and something out. Plugged in, carefully powered up, valves lit up, all seemed good. After a minute or so I noticed a faint smell and noticed a very subtle wisp of smoke. Killed it, looked over and the only obvious damage is to the 5k1 anode resistor on the EL84, which is burnt. Everything else looks fine, there might be a slight discolouration to the cathode resistor but it's so subtle I can't tell.

I tested the 5k1 and it's still perfectly in spec. All resistors are 1watt metal films. Does it need something bigger or could I have made an error elsewhere?

Cheers
Lee

 

[PRR]

> smoke

I've lost track. Assume a 300V supply, tube takes 1/3rd to 2/3rd, 5.1K takes as much as 2/3rd of 300V or 200V.

Do math. 200V^2/5100 is 8 Watts. This may possibly need to be a 16W-20W part.

Not a shock. That is a 12W tube, and resistor-loaded the resistor probably takes a near-equal part of the power. We are probably not working all the way to 12W, true. But this isn't some puny preamp stage.

 

[untune]

> smoke

I've lost track. Assume a 300V supply, tube takes 1/3rd to 2/3rd, 5.1K takes as much as 2/3rd of 300V or 200V.

Do math. 200V^2/5100 is 8 Watts. This may possibly need to be a 16W-20W part.

Not a shock. That is a 12W tube, and resistor-loaded the resistor probably takes a near-equal part of the power. We are probably not working all the way to 12W, true. But this isn't some puny preamp stage.

Thanks PRR, didn't even occur to check - serves me right!

B+ should be around 260V max, and by that calculation I get ~6.4W if I lower to 4K7.  Welwyn do a 14W wirewound that should handle it

 

[Joechris]

I think there is a cap missing on top of the tone pot in your drawing.
Looks like the pot just short the signal to ground.

 

[untune]

I think there is a cap missing on top of the tone pot in your drawing.
Looks like the pot just short the signal to ground.

Originally I did but the tone pot should form a first order high pass filter with the 0.1uf coupling cap, if the pot is switched in; otherwise it just blocks dc from the first stage

 

[untune]

Just a small update; a 10W 4K7 wirewound resistor solved the issue and everything runs stable. First test had horrendous buzzing (due to me not earthing everything properly in a rush to see if everything worked) but I had some time last night to replace inputs with plastic jacks to isolate the them and rearrange the grounding scheme. Much quieter - there is still a low level hiss but it isn't in an enclosure yet, and the valves are very microphonic. Heaters are currently referenced to 0v via artificial centre tap, but I'm wondering if referencing to the EL84 cathode might improve things a bit.

More than enough gain for bass guitar just into v2, so I'm thinking I might change the switching scheme to swap between the mic into v1, or line post v1 but before the vol/tone control if this won't cause impedance issues.

Next step is to get the mic transformer in and test that, then it's just a case of tweaking!

 

[untune]

Another small update in order, I thought!

Been playing and tweaking and everything works nicely. I got the mic tranny in but I'm getting a load of buzz which I can't seem to get rid of (which is a shame because if you ignore that, it actually sounds quite good!) Probably too much gain with two 6au6's, even as triodes - so what I might do is have v1 just for mic/padded line and v2 for direct input from bass (which also sounds great!) etc, and put a switch in to select which of these two goes into the EL84. That way each pre valve is isolated from the noise contributed by the other. Is it safe to do it this way?

Generally it sounds really good, and a 1k pot across the output transformer gives a bit of creative control over final output level and distortion. I did notice however that my HT is way over what I was expecting - I had planned for 250V-260V at V3 (going off what the original Akai schematic claimed at that point) but it's actually 354V! Down to 190V at the V3 plate (no wonder that 1W resistor went up in smoke..)

My dropping resistors seem right on the money, dropping about ~35V per stage. But that still leaves me with a HT of 284V at the first preamp valve. So I'm about 100V higher than I need after the pi filter - is there an ideal way to overcome this or is it another resistor?

I also never considered any other 7 pin miniature valves other than the 6AU6... I found I have a 6C4 which is apparently one half of a 12AU7, mu of 18 or 20 if I recall. Worth experimenting?

Cheers!

 

[mjrippe]

I don't have time to read through five pages of circuit revisions, but two possibilities for the increased B+ pop to mind:

1 - You are using fewer tubes than the original amplifier.  A reduced load on an unregulated power supply will result in a higher voltage.

2 - Many Akai power transformers have primary taps for 100vac (Japan) and 117vac (USA).  If you are using the 100vac primary your secondary voltage will be high.

 

[untune]

I don't have time to read through five pages of circuit revisions, but two possibilities for the increased B+ pop to mind:

1 - You are using fewer tubes than the original amplifier.  A reduced load on an unregulated power supply will result in a higher voltage.

2 - Many Akai power transformers have primary taps for 100vac (Japan) and 117vac (USA).  If you are using the 100vac primary your secondary voltage will be high.

Thanks for your contribution! Indeed you're right, I've pulled the 6AR5 oscillator as it was serving no purpose. I had no use for the additional socket so I wired up the heaters just in case I needed it further down the line. This will account for some of it.

Again yes, I'm using the 100v primary on the transformer (as this was the way it was wired when I got it)  and I'm using  a 230v/110v 100va external step down for UK mains. I'll try the 117v tap and measure the voltages again

Cheers
Lee

 

[mjrippe]

Hi Lee,

I missed that you are in the UK, sorry about that.  However in my experience there are no true 230/110 converters , they are all wired 2:1.  So your 230/110 is likely putting out 115vac or more, depending on the load.

Best,
Mike

 

[untune]

Hi Lee,

I missed that you are in the UK, sorry about that.  However in my experience there are no true 230/110 converters , they are all wired 2:1.  So your 230/110 is likely putting out 115vac or more, depending on the load.

Best,
Mike

Hi Mike,

No problem at all - you're probably right, it's probably not the best converter (it was given to me along with a US Shure M67 mixer) but it does the job!

Late last night I swapped the live over to the 117V tap as opposed to the 100V tap - I'm now seeing a reduction of 51V at the HT of my EL84 which is a big improvement.  I got a measurement of 303V, which is still about 50V higher than I designed for but I noticed the audible hum from the transformer seems to have reduced considerably (might have gone altogether) so, although I've not had time to do a sound test, I'm also hoping noise/hum will have been reduced too.

I'd still like to drop another 40-50V ideally to get things to the points I designed for, is there a safe way to do it?

Cheers!
Lee

 

[untune]

Noise hadn't lowered too significantly, a but NOS rectifier and 2nd stage 6AU6 made for a drop of about 6db which was nice. 1st stage (mic) still buzzes, but not as badly - still not useable though. I'm going to use good shielded wire up to the transformer and ground the input XLR at the input, following ruffrecords' Grounding 101 guide I found on his site last night

At this point, excluding the 1st stage it actually makes a fantastic sounding bass preamp - instrument plugs directly into the 2nd stage, volume is adjusted on the bass itself and the combination of that and a 1K pot across the OT secondary allows for tweaking between clean (lower input, max output) and dirty (max input, lower output).

I'm still leaning towards completely isolating V1 and V2 and making them switchable before the EL84. This way V1 would be a switchable mic or line input with variable volume and tone (high pass) control whereas V2 is a pure instrument amplifier.

I noticed last night that my heaters are measuring 6.7-6.8v rather than 6.3v. Within spec, but would it be a good idea to try to bring this back down? I also have a currently unused 5.7v tap which is intended to be used for a lamp, eventually. Might be best to leave the HT ~300V if it's not going to cause any issues.

Cheers
Lee

 

[ruffrecords]

Finally got my internat sorted out!!

What mic transformer are you using?

Cheers

Ian

 

[untune]

Finally got my internat sorted out!!

What mic transformer are you using?

Cheers

Ian

Hi Ian! Good to hear you're back, have you moved?

I'm using the Sowter 9610; I was very hasty in hooking it up because I was getting impatient so I regret to say I did a sloppy job of it - Curently I've just soldered the relevant wires from the transformer (blue/orange at the input side if I remember) to the XLR. They're neither twisted nor shielded which won't help. The XLR pin 1 is grounded to an input jack (plastic and isolated from the chassis) ground lug which then goes to the 0v ref for the first stage (neg side of the cathode bypass cap and resistor if memory serves) then to a single point on the chassis. This should really be earthed to the chassis at the input I believe.

The mumetal can itself is mounted directly to the chassis and therefore isn't isolated. The black ground wire from the Sowter goes to the chassis directly, and the as for the output wires (red and yellow) the red goes to one side of a SPDT switch which connects to the valve grid via shielded cable and the yellow goes to the 0v reference as stated earlier. Again, I've neither shielded nor twisted these wires yet. The green centre tap (for phantom I think?) is just capped off and pushed out of the way. The buzzing noise I'm experiencing is only present when switched the the microphone input. I'm sure its' just a bad grounding setup on my part coupled with poor lead dress.

Another thing to bear in mind is that this isn't get contained inside an enclosure - it's on the to-do list!

Cheers
Lee

 

[ruffrecords]

Hi Ian! Good to hear you're back, have you moved?

Half moved. we are in rented accommodation while our new place is finished.

IThe XLR pin 1 is grounded to an input jack (plastic and isolated from the chassis) ground lug which then goes to the 0v ref for the first stage (neg side of the cathode bypass cap and resistor if memory serves) then to a single point on the chassis. This should really be earthed to the chassis at the input I believe.

Yes, it should the point is to direct interference (hum) to the chassis instead of direct to the signal 0V.

The mumetal can itself is mounted directly to the chassis and therefore isn't isolated.

Cheers
Lee
[/quote]

That is fine.

Cheers

Ian