Akai 707 Preamp Modification

[ruffrecords]

It dawned on me yesterday that I can't just wire straight into the power transformer (117v pri) which people often do with these units in the States...  I'll have to keep the original voltage selector and step down in place too, unless I opt for an external step down... I do have one somewhere. Any advantages to that?

It is always a PITA working US gear in the UK. There is no simple solution. The best answer long term solution is to replace the mains transformer. I have a Maplin step down transformer that I use for US gear.

I'm still in the midst of updating the schematic, removing the necessary components etc. Hoping to get it done in the next few days, time permitting. I was looking at inserting the DI input between the volume pot and 2nd 6au6. Also pondering if it might be interesting to make it switchable so it can be inserted at either the first or second tube stage.

Because a DI input is high impedance it is susceptible to picking up interference. You should therefore keep connections to it as short a possible. Switching from on stage to another will make this difficult .

In terms of the tone control I might go for a variable HPF, and rework the switch to bring it in or out.

Sounds good.

I'll have to do some serious reading up in regards to the output transformer as it's all very new to me. Thanks for the info PRR, I just have to work out how it all applies I'm also wondering if I might be able to insert an output level control, so that I can overdrive the input and attenuate the output.

PRRs advice was very good. You don't need the power  the stage can deliver at present and his method will allow you to use a good quality pro audio transformer.

An output control is straightforward. It can be as simple as a 1K log pot connected across the transformer secondary.

Cheers

IAn

 

[untune]

Thanks Ian

Apologies for the delay replying, been busy with work and other commitments but finally cleaned the schematic up (to a degree!)

It is always a PITA working US gear in the UK. There is no simple solution. The best answer long term solution is to replace the mains transformer. I have a Maplin step down transformer that I use for US gear.

I think for the purposes of testing and development, I have an external transformer from another piece of US studio gear. The power in the Akai goes through the voltage selector in the first transformer then into the next which has a 117v and 100v - it's wired up to the 100v. Is it ok to stick to this one? I could potentially mount the power selector/tranny in an external box and run it like that, I suppose?

Because a DI input is high impedance it is susceptible to picking up interference. You should therefore keep connections to it as short a possible. Switching from on stage to another will make this difficult .

Noted, thanks - because of the arrangement of the predrilled holes in the front panel and the proximity to V1 and V2, it will be relatively easy to create a switched path that doesn't travel very far at all and should stay away from power - I do have some low capacitance shielded cable somewhere from a contact microphone project which might be ideal.

PRRs advice was very good. You don't need the power the stage can deliver at present and his method will allow you to use a good quality pro audio transformer.

It sounds like this is the way to go - I'll look into this option

An output control is straightforward. It can be as simple as a 1K log pot connected across the transformer secondary

Easier than I thought - might this cause any issues with impedance at the output?

Ok so attached is a *rough* starting point if nothing else - I've tried to clear away the irrelevant parts and keep things as efficient as possible. I've not yet incorporated any ideas in regards to wiring the pentodes as triodes or the negative feedback etc yet - I want to make sure things are going in the right direction before making any design changes.

Firstly, I noticed V1 is on it's own 5.7V tap on the transformer - the lower voltage I assume reduces gain somewheat and helps the tube live longer. I read a suggestion that it might be a good idea to disconnect the unused 6AR5 (0.4A draw), move all the other valves onto the 6.3V/1.8A tap (0.3+0.3+0.76=~1.36A) and run the VU lamp off the 5.7V/0.3A , keeping it separate from the audio valves and possibly prolonging the life of the bulb. The 6X4 has it's own 0.6A tap. What do you think?

I've managed to track down an old Sifam signal meter, looks like the type from the old Series 2 Ferrographs - I won't know if it's working until it arrives, but is the VU in the right place (ie on one side of the balanced output, post transformer) or is there a better place to run it?

Apologies for the bombardment of questions but it's all starting to sink in, one step at a time!

Cheers
Lee

 

[ruffrecords]

I think for the purposes of testing and development, I have an external transformer from another piece of US studio gear. The power in the Akai goes through the voltage selector in the first transformer then into the next which has a 117v and 100v - it's wired up to the 100v. Is it ok to stick to this one? I could potentially mount the power selector/tranny in an external box and run it like that, I suppose?

This is not clear to me. You just need to be sure the AKAI gets its expected mains voltage.

Noted, thanks - because of the arrangement of the predrilled holes in the front panel and the proximity to V1 and V2, it will be relatively easy to create a switched path that doesn't travel very far at all and should stay away from power - I do have some low capacitance shielded cable somewhere from a contact microphone project which might be ideal.

The cable definitely needs to be shielded!!

Easier than I thought - might this cause any issues with impedance at the output?

It is a fairly common solution. You could wire a resistor across the 1K pot to make it look closer to 600 ohms. The transformer will not mind and it will not make a lot of difference to the EL84 if wired as PRR suggests. As long as you run it into a 'modern' 10K bridging input it will be fine.

Ok so attached is a *rough* starting point if nothing else

Attachment does not appear to have worked

Firstly, I noticed V1 is on it's own 5.7V tap on the transformer - the lower voltage I assume reduces gain some wheat and helps the tube live longer. I read a suggestion that it might be a good idea to disconnect the unused 6AR5 (0.4A draw), move all the other valves onto the 6.3V/1.8A tap (0.3+0.3+0.76=~1.36A) and run the VU lamp off the 5.7V/0.3A , keeping it separate from the audio valves and possibly prolonging the life of the bulb. The 6X4 has it's own 0.6A tap. What do you think?

Those seem good ideas to me. Tubes are designed to run on 10% tolerance heater supplies so they can legitimately go below 5.7V but they do run best at 6.3V

I've managed to track down an old Sifam signal meter, looks like the type from the old Series 2 Ferrographs - I won't know if it's working until it arrives, but is the VU in the right place (ie on one side of the balanced output, post transformer) or is there a better place to run it?

The natural place to add the VU is to the transformer secondary. However, genuine VU meters do add distortion to the line they are attached to due the acting as a non-linear load (caused by the internal bridge rectifier). You may like the added 'tone' this creates but if you do not then you will need to add a buffer circuit.

Cheers

Ian

 

[untune]

Apologies, it was a little confusing; if you look at http://i905.photobucket.com/albums/ac253/rvergini/Akai004.jpg you'll see the transformer in the unit that feeds the heaters and the amp, etc. The heater taps are visible at the bottom. Top right is the 250v-0-250v that feeds the 6X4 rectifier, and on the left there are three pins 0-100v-117v. In mine, there's a wire connected to the 100v tag which runs from an octal plug, which feeds this transformer from the motor circuit and the mains transformer (which has switched selectable voltage - 100, 110, 117, 200, 220, 230v). When these units are modded in the States, they hook up a fuse and switch to that 117v and 0 pin. Of course I can't do that being in the Uk, but in mine the 100v is the one that's hooked up - I'm just curious what the significance is

Practically everything I could pass it into will be 10k I'd imagine, so it shouldn't be a problem.

Thanks - that will make the heater wiring much neater, should keep hum down. As long as disconnecting the 6AR5 doesn't have any negative effects - not sure if the concept of headroom applies to heater current in this sense? Re the VU that's what I assumed - I suppose I can try it in place and see - if not I will look as putting a buffer in. Perhaps the spare tube socket could play a part.

Also - the attachment failed on first posting and I quickly corrected it, but you may have missed it - let me know if it's available now, otherwise I'll try an alternative

Cheers
Lee

 

[ruffrecords]

The attachment works fine now. Couple of comments on the schematic.

The instrument input goes to the wiper of the volume control. This means it will short out the instrument input if it is turned down and it will be loaded by the output of the first stage when turned up. The usual way to do an instrument input  is to use additional contacts on the jack socket to disconnect the first stage when the instrument is plugged in. This usually means there is a screened cable from the output of the first stage to the jack socket and another from the jack socket back to the volume control.

You have a strange schematic symbol fo the 6BQ5. It is a pentode just like the 6AU6 so it should have the same symbol. You also seem to have its anode shorted to the previous 6AU6 anode and no dc connection to the screen grid.

Cheers

Ian

 

[untune]

Thanks again for taking the time, Ian

I had a crack at updating based on your comments today inbetween working (I've been doing this very piecemeal so please forgive any inadequacies) and I've attached the result.  I'm slowly figuring out the hows and whys of each stage as I go along and learning an awful lot.

Firstly, I see your point regarding the inst. input - I think I've corrected this properly, ie completely disconnecting the first stage upon inserting a jack into J4, removing the loading problems with the volume pot. I might have to forgo a 1st stage/2nd stage switch lest the switching scheme get a bit too complicated - something I can always look into later down the line. I believe this would leave me with fixed gain on the instrument input, but then the instrument could always be the point of attenuation a bit like that classic direct amplifier from the Motown studio I'm not 100% sure about the values of R4 and R8. Do these act as the grid leak/grid stopper and will they cause any issues when the jack is disconnected?

Apologies for the incorrect 6BQ5/EL84 symbol - every schem I'd seen (including the Akai one in the first post) had the internal supressor/cathode connection marked up and that was the only one available which seemed similar. I've changed it for a regular pentode and made the connection on the outside of the tube.

The shorted anode etc was my fault - got wires crossed (literally) when removing the components you marked up earlier to eradicate the EQ network. I removed what was previously C11 (0.002uF) and connected the screen grid above C10 - is this correct?

Also, I picked up a suggestion (although I forget where from) that C11 (previously C12) could be upped to 40uF. At present it's a 20+20uF can along with C13, I was going to replace it with separates. Any advantage in upping the value?

Cheers!
Lee

 

[ruffrecords]

Firstly, I see your point regarding the inst. input - I think I've corrected this properly, ie completely disconnecting the first stage upon inserting a jack into J4, removing the loading problems with the volume pot. I might have to forgo a 1st stage/2nd stage switch lest the switching scheme get a bit too complicated - something I can always look into later down the line. I believe this would leave me with fixed gain on the instrument input, but then the instrument could always be the point of attenuation a bit like that classic direct amplifier from the Motown studio I'm not 100% sure about the values of R4 and R8. Do these act as the grid leak/grid stopper and will they cause any issues when the jack is disconnected?

R4 is the grid leak ad 1M is an OK value for it. R8 is a grid stopper. Rather a large value for a stopper and more like the kind of values used in guitar amps to curtail high frequency response using the Miller effect - except pentodes don't have a Miller effect - that is sort of why they were invented. Anyway you can leave it in. All it will do is worsen the noise slightly.

Apologies for the incorrect 6BQ5/EL84 symbol - every schem I'd seen (including the Akai one in the first post) had the internal supressor/cathode connection marked up and that was the only one available which seemed similar. I've changed it for a regular pentode and made the connection on the outside of the tube.

Understood

The shorted anode etc was my fault - got wires crossed (literally) when removing the components you marked up earlier to eradicate the EQ network. I removed what was previously C11 (0.002uF) and connected the screen grid above C10 - is this correct?

Yes. the circuit is basically OK now.

Also, I picked up a suggestion (although I forget where from) that C11 (previously C12) could be upped to 40uF. At present it's a 20+20uF can along with C13, I was going to replace it with separates. Any advantage in upping the value?

Upping the value will reduce HT ripple and hence the amount of hum in the output. For tape recorder of that era, a 40dB signal to noise ratio was good. For a mic pre we want to do a lot better. However, the 6X4 has a maximum peak current draw that must not be exceeded which is why C12 must not be more than 20uF. It is a little more difficult to work out how much bigger C11 can safely be made but 40uF should be OK. C10 and C1 can also usefully increased to sya 47uF.

Cheers

ian

 

[Gene Pink]

Are you guys OK with J-2 Hi Z mic input directly driving the secondary of T-1?

Gene

 

[PRR]

> strange schematic symbol fo the 6BQ5

Hah! Filamentary pentode-diode! Have not seen that in a while! (Odd even for battery radios, a low-Mu triode is usually ample after the detector diode.) If he could even find such a tube, the diode plate is a terrible cathode.

> internal supressor/cathode connection marked up

"Usually" we leave this detail to final assembly. With years of experience we "know" which tubes have a G3 pin and what to do with it. Agree that as a beginner you should probably make note. FWIW I have seen power pentodes work fine with G3 floating (possible on EL34). I do not know if this is stable in heavy abuse. 6BQ5/EL84 does NOT give you a choice, and the internal cathode strap is fine and not always documented in sketches.

{edit- add NOT in last lines}

 

[ruffrecords]

Are you guys OK with J-2 Hi Z mic input directly driving the secondary of T-1?

Gene

Not really. They were in the original tape recorder schematic. I think the OP may just have forgotten to delete.

Good catch.

Cheers

Ian

 

[untune]

R4 is the grid leak and 1M is an OK value for it. R8 is a grid stopper. Rather a large value for a stopper and more like the kind of values used in guitar amps to curtail high frequency response using the Miller effect - except pentodes don't have a Miller effect - that is sort of why they were invented. Anyway you can leave it in. All it will do is worsen the noise slightly.

Thanks Ian - I'm concentrating more on getting the topology correct first, then I can focus on tweaking values. For example - you suggested triode strapping the pentodes which seems like my best option, I've just yet to incorporte that until I'm confident anything still tape-recorder related has been eliminated. I fully expect there are better values the grid leak and stoppers in this position but considering that inst. input is primarily going to be a bass guitar in, I used those commonly found in amps of that nature. How this in conjunction with the vol. pot will load the anode of the previous stage, and as such how it will play with the previous stage in general, is something I'm still getting my head around  I imagine as soon as those 6AU6's are strapped as triodes, a lot will change in terms of component arrangement and values... but it will be easier to modify this schematic than the original.

The circuit is basically OK now. Upping the value will reduce HT ripple and hence the amount of hum in the output. For tape recorder of that era, a 40dB signal to noise ratio was good. For a mic pre we want to do a lot better. However, the 6X4 has a maximum peak current draw that must not be exceeded which is why C12 must not be more than 20uF. It is a little more difficult to work out how much bigger C11 can safely be made but 40uF should be OK. C10 and C1 can also usefully increased to say 47uF.

Thanks for these suggestions - I've made notes and will factor in. I re-read the whole thread last night and much more of it makes sense now.

Are you guys OK with J-2 Hi Z mic input directly driving the secondary of T-1?

Good catch - I pencilled the XLR/transfomer in to the original schematic and didn't notice when I redrew it. I could, I assume, connect the transformer sec to the switched contact of the hi-z mic input to disconnect when the hi-z is used but then the line input would become a problem. I could just put an SPST to isolate the balanced input when required - hardly elegant but then I don't have to give up the hi-z mic in. The other option is to do away with the hi-z in, and use the spot on the front panel for a mic-related switch - impedance selection perhaps, depending on the transformer used.

"Usually" we leave this detail to final assembly. With years of experience we "know" which tubes have a G3 pin and what to do with it. Agree that as a beginner you should probably make note. FWIW I have seen power pentodes work fine with G3 floating (possible on EL34). I do not know if this is stable in heavy abuse. 6BQ5/EL84 does give you a choice, and the internal cathode strap is fine and not always documented in sketches.

Of course - I'm picking up information from all over and don't have the benefit of that experience, but as long as the symbol is clear for now - when I have a final schem I'll manually add that internal strap! Thanks for your earlier suggestion regarding the 6BQ5 and running it as a resistance-loaded triode. I'm going to read up on this properly when I get a chance. Out of interest, what kind of effect does using the ungapped tranny as opposed to gapped have on the output , low frequency distortion etc?

 

[ruffrecords]

Out of interest, what kind of effect does using the ungapped tranny as opposed to gapped have on the output , low frequency distortion etc?

A gapped transformer is necessary if it is required to carry a dc current as this prevents the core from saturating. This is the case when the transformer is directly in series with the anode of the 6BQ6 as it is now. The effect of the gap is also to signficantly reduce the inductance of the transformer which directly affects the bass response, For good bass response with a gapped transformer you need a large core and more turns to increase the inductance.  If you use a resistor load in the anode circuit and ac couple it to a transformer then there is no dc current flowing and you can use an ungapped transformer. Having no gap allows a given size core to have a higher inductance and a better bass response.

Cheers

Ian

 

[untune]

A gapped transformer is necessary if it is required to carry a dc current as this prevents the core from saturating. This is the case when the transformer is directly in series with the anode of the 6BQ6 as it is now. The effect of the gap is also to signficantly reduce the inductance of the transformer which directly affects the bass response, For good bass response with a gapped transformer you need a large core and more turns to increase the inductance.  If you use a resistor load in the anode circuit and ac couple it to a transformer then there is no dc current flowing and you can use an ungapped transformer. Having no gap allows a given size core to have a higher inductance and a better bass response.

Thanks Ian, that's a very clear explanation I suppose then that the preceding stage (ie what will eventually be a triode 6BQ5) will have an impact on the choice of transformer. I've got the VTB 2291 noted down as a starting point for now.

Do you think it's a good idea to lose the hi-z mic input given the issues it might cause, and have the line input switching contact instead wired to break the connection to the input transformer secondary? Perhaps it'd make more sense to make a simple hi to lo-z transformer box and use that to drive the old hi-z dynamics into the XLR input

 

[ruffrecords]

Do you think it's a good idea to lose the hi-z mic input given the issues it might cause, and have the line input switching contact instead wired to break the connection to the input transformer secondary? Perhaps it'd make more sense to make a simple hi to lo-z transformer box and use that to drive the old hi-z dynamics into the XLR input

The line input is heavily attenuated before being amplified - a pointless really. Better to feed it into the second stage - in other words the instrument input can double as a line input.

Whether or not to retain the hi-z mic anput is up to you. The Classic Solo mic pre project can have a DI input wired to the first stage input after which there is as much as 50dB of gain. This is no different and it wired carefully it should be OK.

Cheers

Ian

 

[untune]

Thanks Ian - yeah I thought the same, the voltage divider on the input can be altered though - I've just kept original values in place for now.  I had a think about t last night and I decided to remove the hi-Z mic in (not much advantage really - I'm better off modifying the mics themselves, or converting them to lo-z at the mic end to drive longer cables) and the hole for the jack can be used for a toggle switch to select between mic and line input. This keeps the two inputs completely isolated and should avoid any compromises.

I see what you mean about inserting at the second stage - what I'm going to do is keep the DI into the 2nd stage and keep the line input at the 1st with greater attenuation. That way I have a choice of running through the 1st for tone shaping or additional saturation, or the 2nd for a cleaner input.

I've attached a new schematic with the changes; the tone circuit I'm still not sure about but I'm going to leave that until later - although I'm interested to know how it interacts with the DC blocking cap (C5) - might need to look at a more complex switching scheme. After reading into some smoothing cap discussions I'll up the values of C1 and C10 as per your suggestion, leaving C12 at 20uF to keep the rectifier happy. R15 in the original circuit has suffered some heat damage (but still tests perfectly in spec) so I'm tempted to replace that, and perhaps R14, with high wattage resistors to be on the safe side.

My next goal is to work out what needs to be done to get each stage operating as a triode, and how it will affect the resistors and bypass cap etc. So starting with V1 - Is it as simple as just connecting screen and anode via a resistor, or is there additional rerouting that needs to take place?

Cheers
Lee

 

[ruffrecords]

My very first mic pre design used a 6AU6 wired as a triode. From memory (it was about 10 years ago) I used a 39K anode load and a 390 ohm cathode resistor. I think I wired the anode, screen grid and suppressor grid together. I remember there was a design that used the screen grid as the anode and grounded the anode. This was because is screened the screen grid from the heaters and resulted in less hum. I think the circuit was in the Radio Designers Handbook. I will see if I can find it.

Cheers

Ian

 

[AndreasSchwarz]

Hello Ian,

pls.

Regards Andreas

 

[PRR]

> used the screen grid as the anode and grounded the anode.
> I think the circuit was in the Radio Designers Handbook.

I recall it as a BBC remote mixer. Tons of gain on small battery.

 

[ruffrecords]

Hello Ian,

pls.

Regards Andreas

I get an attachment not found error.

Cheers

Ian

 

[AndreasSchwarz]

from the web:

http://audiophool.com/Misc/6AU6%20grounded%20plate.pdf