I breadboard this portion of the circuit. I could adjust the voltage across R3 but not R4.
Next I took both jfets out and replaced them with other jfets that measured about 12mA , and I could adjust both R3 and R4 to above 5 Volt.
Therefore I think low Idss maybe the problem
Next I took both jfets out and replaced them with other jfets that measured about 12mA , and I could adjust both R3 and R4 to above 5 Volt.
Therefore I think low Idss maybe the problem
With the 7.8mA jfets, when I disconnect the V+ supply I was able to adjust the voltage of R4, that is the pot of P channel jfet.
This is really puzzling me.
Two jfets in circuit, only R3 can be adjusted.
Really need to understand why before I proceed to solder in the 12mA(not so closely matched) jfets.
Hope members can explain
Thanks
This is really puzzling me.
Two jfets in circuit, only R3 can be adjusted.
Really need to understand why before I proceed to solder in the 12mA(not so closely matched) jfets.
Hope members can explain
Thanks
How did your preamp cause your F5 to malfunction? Perhaps that will give a clue.
You could check the solder joints of the new components and of the components in and around the front end. Perhaps a bad solder joint is the cause of the problem.
There are not too many components that are likely to be damaged. Since you have replaced all of the JFETs, transistors, and mosfets, double check that there are no mix-ups in locations.
You could check the solder joints of the new components and of the components in and around the front end. Perhaps a bad solder joint is the cause of the problem.
There are not too many components that are likely to be damaged. Since you have replaced all of the JFETs, transistors, and mosfets, double check that there are no mix-ups in locations.
Noise and spikes from lose connection to preamp. P channel Jfet damaged.
I can get the amp to work with 12mA Jfets but not with 7.8mA Idss.
That is why breadboard that portion to find out the mystery.
Thanks
Attachments
Yes done already.
Disconnected V+ to test P jfet
and and disconnected V- to test N jfet
go back with P1 and P2; confirm that measuring across R3 and R4, you want minimum ohms there
replace R3 and R4 with 4K7
that will cover whatever Idss range of JFets you're using and - if everything else is proper, you'll be able to set everything
Are your jfets working? Since removing the jfets from the PCB, have you tested them individually, say just for Idss, and not in that partial circuit?
As others have noted, a working pair of 7.8mA IDSS jfets should not have trouble in an F5.
As others have noted, a working pair of 7.8mA IDSS jfets should not have trouble in an F5.
Yes R3 R4 increased to 5K1
and P jfet j74 still not conducting.
Jfets Idss tested again and all about 7.8/9 mA. That means they are not damaged. Also tested with 1K resistor and led.
Maybe need to modify input part for both jfets to work
Thanks
G'day All
I'd like to remove the current limiter on my F5, reason being I've some Thiel 2.3's that are current hungry so I'd like to see what sonic benefit the removal brings. Is there anything to keep an eye on other than re-biasing?
Also, I've a 24V secondary tranfo I'd like to try to bring the output up. At the moment I dont have higher wattage resistors to change out R9 through R12 as suggested. Would it be safe not changing them for now so long as I dont bias to high? Additionally when upping the rail voltage, how will this effect the output into 4 ohm please?
Many Thanks
I'd like to remove the current limiter on my F5, reason being I've some Thiel 2.3's that are current hungry so I'd like to see what sonic benefit the removal brings. Is there anything to keep an eye on other than re-biasing?
Also, I've a 24V secondary tranfo I'd like to try to bring the output up. At the moment I dont have higher wattage resistors to change out R9 through R12 as suggested. Would it be safe not changing them for now so long as I dont bias to high? Additionally when upping the rail voltage, how will this effect the output into 4 ohm please?
Many Thanks
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The limiter can be removed easily. The F5T doesn't have it, so it's not needed.
24V transformer will give you about 32V rails. This is about the highest I'd be comfortable running Toshibas with no cascode, and I'd not run Linear Systems Jfets that high.
4ohm loads are more demanding of current than voltage swing, and increasing the rail voltage doesn't do all that much for current, but it is likely to be a little better, so it's worth trying.
24V transformer will give you about 32V rails. This is about the highest I'd be comfortable running Toshibas with no cascode, and I'd not run Linear Systems Jfets that high.
4ohm loads are more demanding of current than voltage swing, and increasing the rail voltage doesn't do all that much for current, but it is likely to be a little better, so it's worth trying.
Pretty sure I bought Toshiba's, will have a look & order some if not
Whats your thought please with leaving R9~12 standard wattage for now?
The 24V secondary transfo is 600VA, not over the top but surely enough legs with the limiter gone
By the way I now have a Supratek 6SN7 driving the F5, a very nice match indeed
Will post findings once the F5 changes are in place. Thanks 6L6
AL
Whats your thought please with leaving R9~12 standard wattage for now?
The 24V secondary transfo is 600VA, not over the top but surely enough legs with the limiter gone
By the way I now have a Supratek 6SN7 driving the F5, a very nice match indeed
Will post findings once the F5 changes are in place. Thanks 6L6
AL
having a hard time finding metal film 5W 100R 1% resistors to upgrade R9~12 on the F5.
From what I can make out the stock 3W Vishay's the F5 kit comes with are metal film, should I stick with metal film or another resistor type is ok to use please?
From what I can make out the stock 3W Vishay's the F5 kit comes with are metal film, should I stick with metal film or another resistor type is ok to use please?
I removed the current limiter & protection then re-biased my F5;
24v rail's
P3 centred R3Ω = R4Ω w/- amp off
600mV with ~0mV offset
I then measured across R5 & R6 and got 4.92 and 5.2v.
Is this acceptable and whats possibly causing the imbalance when my bias voltage's are equal please?
24v rail's
P3 centred R3Ω = R4Ω w/- amp off
600mV with ~0mV offset
I then measured across R5 & R6 and got 4.92 and 5.2v.
Is this acceptable and whats possibly causing the imbalance when my bias voltage's are equal please?
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