Never ever stop, its knowledge that are forgotten, i appreciate all your time and effort but more over the knowledge we gain as a audience cant be measured ! thumbs up!
Great to see you again! You sound well. BOY that cap was SHORTED!!!! Guess the 1.8K resistor acted like a sacrificial fuse and saved everything else. The unloaded voltage sure went above the rated working voltage of those caps too. That was a smart thing to do... changing both of them out for higher voltage caps. Cheap insurance and long life. Take care of yourself and post if you want to and when you want to! It is always good to see you!
Seriously, you e taught me, and I'm sure countless others knowledge that is disappearing. Between you and Mr Carlson I've learned so so much about vacuum tubes and application. Thank you Sir.
I am so glad you take us along on your journeys with amplifiers! You have taught me a lot, I am absolutely in love with building my own tube amps. I have now built several with wonderful success :)
I have to echo the other comments. I've learned a lot from you and always enjoy your videos, please keep them coming. You have a dedicated set of followers!
Well done, an excellent diagnosis. The schematic in the chassis is a great move, always good to open a bit of gear you have built and be impressed how smart the younger you was! In a guitar amp with a similarish situation I used a MOSFET as a series element with a long time constant in the gate circuit so that the HV to the small signal stages ramped up slowly to a predefined level. In this case though I think you have a simple and permanent fix.
Your videos have been a huge inspiration for me. Just got my Tek TM504 and SG505. The AA501 [opt 1] should be here Friday. Also just pulled the trigger on a DM501a for the last slot. Just wanted to let you know, out of the many UA-cam video channels, I have enjoyed yours the most.
The SG505 and AA501 are the ones to have. Mine have been flawless for the years I have had them (knock-on-wood...) I always wanted that DM501 also but ended up getting other meters. I am pleased you enjoy my videos. I will try to make a few more. Stay safe.
I really know nothing about circuits. Wish I did.. But I gain more understanding every time I watch. So my question, why wouldn't you install a thermistor coming off of the fuse to not immediately shunt full voltage at the moment of power on? Thanks for helping my journey.
Thermisters are good to use in the AC power line. I have used them many times and they will reduce inrush current. As for preventing over-voltage on electrolytic capacitors when using solid state rectifiers , they may not help so much as there is no load on the power supply in a vacuum tube amplifier until the filaments warm up but overall they are often a good idea to use.
Modern elyts usually don't like any overvoltages even for a short time. BTW, the tube layout seems to be asymmetrical. Is there any particular reason for that or could it have been built symmetrical? Nice video. 👍
Hello, got a few questions I could use your input on, Would subbing kt88's in a Dynaco st70 work? I have 2 non-linear 120w OT's, dimensions are 4x4x3", and a 4X4X5" PT . Is room enough on a stock dynakit chassis for this iron? Would it be better to fab a chassis with greater separation? Would a different design work better? If you require a patreon donation to bother with answering this many questions I can go up to 50 bucks. Thanks, Lou.
Hi Lou, That is a big question. First off, we must know the filament voltage and current of each tube. In virtually all cases, the filament voltage is 6.3 but the required current is higher in the higher power tubes. Start with a 6V6 - fil current is 0.45A, 6L6GC fil current 0.9A, EL34 fil current 1.5A and KT88 fil current 1.6A. So, from this perspective, the KT88 although it is 100mA more per tube than the EL34, it will probably be OK. Now we have a bias issue. Back to the four tubes above - the 6V6 needs about -15V bias, the 6L6 about -37, the EL34 around -37 similar to the 6L6 and the KT88 more like -55 V bias. These are ball park bias values and must be adjusted to not exceed plate dissipation for each tube type. The point is, if you put a KT88 in the socket of a EL34 and can not adjust the bias to a safe plate current, the KT88 can get way too hot by drawing too much anode current. The funny thing about this is, you may be able to get away by putting a 6V6 in a KT88 amplifier (because the bias is so negative it will keep the 6V6 current low) but you can not put a KT88 in a 6V6 socket for the same reasons, i.e the bias voltage is too low and, and as can be seen from filament current requirements, the filament of the KT88 is almost four (4) times higher. If you try the KT88 in the Dynaco ST70, you should monitor the plate current with something like a bias-probe to make sure anode (plate) current is not exceeded. Hope this helps.
I have a question for you Sir. I want to build an RF amplifier using two 813's for use on the HF bands, my question is what voltage should the plate transformer be?, my math is somewhat sketchy but Iam figuring around 3,200 volt before filtering. Would an intermediate use 3,200 volt transformer be suitable?, I have a seperate filiment transformer put aside already. Would you have an email address i can message you regarding this subject?. Thanks for reading!!!
2500 VDC is about the highest plate voltage you want for a pair of 813. A 3200 volt CT transformer into a full wave center tap rectifier (FWCT) and then into a capacitor filter should give you about 2000 VDC. A large sized microwave oven transformer (MOT) is between 1800 and 2000 volts and into a bridge (four diodes) will end up between 2250 and 2500 VDC with a capacitor filter. However... microwave transformers have one end of the HV winding connected to the frame (ground) of the transformer so that wire will have to be carefully removed and properly insulated - probably not the best idea to use MOT's. If the 3200 you are mentioning above is DC out of a rectifier, that is a bit high even into a fully loaded choke input supply which will drop it to about 90% or 2900 volts. Be sure to use a bleeder resistor on your HV PS and be safe... High Voltages at this level should be considered, Death On Contact. You can find my email address on QRZ.com 73 WA4QGA
Not for nothing my friend but every 2 of amplifier or receiver that I've ever worked on. E I've always removed the tubes before I flipped it upside down. I found it a lot safer for the tubes if I did that because I made the same mistake. You're doing right now moving it around with the tubes still in it. And I've broken a couple of tubes doing it that very. It's not very safe for the tubes which you're moving it around like that
I am surprized, the caps you installed look like the cheep chinese chech & chong brand.they have been known to test good new and then short lived under use.
I think that happened to me twice as the voltage doubler capacitors failed twice. After the last replacements, a few years ago, they have been stable and reliable and run several hours per day.
This is the third time I have had issues with the electrolytic capacitors in this unit. In this case, I think it was a little over-voltaged at start up but on two occasions a few years back, the capacitors in the power supply went bad - I believe opened. The HV power supply is a voltage doubler and nothing is over-voltage in that area - the two larger brown ones in the video. Maybe I changed brands and that is why they have lasted the last few years. Like you say - just junk capacitors. I have electrolytic capacitors in McIntosh amplifiers 50+ years old and they work perfectly. This little amplifier can run 12-15 hours a day at times as I have it on with background music. Thanks for your comments.
Never ever stop, its knowledge that are forgotten, i appreciate all your time and effort but more over the knowledge we gain as a audience cant be measured ! thumbs up!
Seems like you forgot English grammar too. " ' "
Great to see you again! You sound well.
BOY that cap was SHORTED!!!! Guess the 1.8K resistor acted like a sacrificial fuse and saved everything else.
The unloaded voltage sure went above the rated working voltage of those caps too. That was a smart thing to do... changing both of them out for higher voltage caps.
Cheap insurance and long life.
Take care of yourself and post if you want to and when you want to! It is always good to see you!
Thanks for pumping life back into the 7591. It is a wonderful and underrated tube. And don't stop making videos. 😁
Thanks for taking us along on the journey. I watched this while building a 20 watt tube amp.
Seriously, you e taught me, and I'm sure countless others knowledge that is disappearing. Between you and Mr Carlson I've learned so so much about vacuum tubes and application. Thank you Sir.
I am so glad you take us along on your journeys with amplifiers! You have taught me a lot, I am absolutely in love with building my own tube amps. I have now built several with wonderful success :)
I have to echo the other comments. I've learned a lot from you and always enjoy your videos, please keep them coming. You have a dedicated set of followers!
Thanks for sharing the knowledge David - I think I’ve been watching your channel for over ten years now and I always learn something. Be well
David, I always enjoy your videos. Thank you for making them.
Thank you for sharing your knowledge with us 😁
I'm learning a lot from your videos 👍
Please keep making these videos, as they are an inspiration! 👍
Well done, an excellent diagnosis. The schematic in the chassis is a great move, always good to open a bit of gear you have built and be impressed how smart the younger you was! In a guitar amp with a similarish situation I used a MOSFET as a series element with a long time constant in the gate circuit so that the HV to the small signal stages ramped up slowly to a predefined level.
In this case though I think you have a simple and permanent fix.
Very enjoyable, hope you stay around+++++
Love your videos I just made a few amplifiers by hand with your inspiration
Hooray you’re back!!!
Your videos have been a huge inspiration for me. Just got my Tek TM504 and SG505. The AA501 [opt 1] should be here Friday. Also just pulled the trigger on a DM501a for the last slot.
Just wanted to let you know, out of the many UA-cam video channels, I have enjoyed yours the most.
The SG505 and AA501 are the ones to have. Mine have been flawless for the years I have had them (knock-on-wood...) I always wanted that DM501 also but ended up getting other meters. I am pleased you enjoy my videos. I will try to make a few more. Stay safe.
I love repair videos.
fixing our home made gear is fun shoebill 300B stereo class A amp i built runs 8 hours every weekend best amp ever
Please keep making videos 🙏
I noticed that on some pl-259's I was using , thanks for the info !
f&t caps are best now a days.
I agree with you and they come in higher voltage ratings also. Thanks for your comment.
Hi david, i would be concerned about C8 also since in a zero current draw in R19 it also will reach >450V at powerup!
I really know nothing about circuits. Wish I did.. But I gain more understanding every time I watch.
So my question, why wouldn't you install a thermistor coming off of the fuse to not immediately shunt full voltage at the moment of power on?
Thanks for helping my journey.
Thermisters are good to use in the AC power line. I have used them many times and they will reduce inrush current.
As for preventing over-voltage on electrolytic capacitors when using solid state rectifiers , they may not help so much as there is no load on the power supply in a vacuum tube amplifier until the filaments warm up but overall they are often a good idea to use.
Modern elyts usually don't like any overvoltages even for a short time.
BTW, the tube layout seems to be asymmetrical. Is there any particular reason for that or could it have been built symmetrical?
Nice video. 👍
Can you tell us what the pots you are using to balance the signal, and the pot you use for the current balancing? Thank you
Is it a good idea to put an electrolytic capacitor in between two hot 5W resistors ?
Hello, got a few questions I could use your input on, Would subbing kt88's in a Dynaco st70 work? I have 2 non-linear 120w OT's, dimensions are 4x4x3", and a 4X4X5" PT . Is room enough on a stock dynakit chassis for this iron? Would it be better to fab a chassis with greater separation? Would a different design work better? If you require a patreon donation to bother with answering this many questions I can go up to 50 bucks. Thanks, Lou.
Hi Lou, That is a big question. First off, we must know the filament voltage and current of each tube. In virtually all cases, the filament voltage is 6.3 but the required current is higher in the higher power tubes. Start with a 6V6 - fil current is 0.45A, 6L6GC fil current 0.9A, EL34 fil current 1.5A and KT88 fil current 1.6A. So, from this perspective, the KT88 although it is 100mA more per tube than the EL34, it will probably be OK.
Now we have a bias issue. Back to the four tubes above - the 6V6 needs about -15V bias, the 6L6 about -37, the EL34 around -37 similar to the 6L6 and the KT88 more like -55 V bias.
These are ball park bias values and must be adjusted to not exceed plate dissipation for each tube type.
The point is, if you put a KT88 in the socket of a EL34 and can not adjust the bias to a safe plate current, the KT88 can get way too hot by drawing too much anode current.
The funny thing about this is, you may be able to get away by putting a 6V6 in a KT88 amplifier (because the bias is so negative it will keep the 6V6 current low) but you can not put a KT88 in a 6V6 socket for the same reasons, i.e the bias voltage is too low and, and as can be seen from filament current requirements, the filament of the KT88 is almost four (4) times higher.
If you try the KT88 in the Dynaco ST70, you should monitor the plate current with something like a bias-probe to make sure anode (plate) current is not exceeded.
Hope this helps.
I have a question for you Sir. I want to build an RF amplifier using two 813's for use on the HF bands, my question is what voltage should the plate transformer be?, my math is somewhat sketchy but Iam figuring around 3,200 volt before filtering. Would an intermediate use 3,200 volt transformer be suitable?, I have a seperate filiment transformer put aside already.
Would you have an email address i can message you regarding this subject?. Thanks for reading!!!
2500 VDC is about the highest plate voltage you want for a pair of 813. A 3200 volt CT transformer into a full wave center tap rectifier (FWCT) and then into a capacitor filter should give you about 2000 VDC. A large sized microwave oven transformer (MOT) is between 1800 and 2000 volts and into a bridge (four diodes) will end up between 2250 and 2500 VDC with a capacitor filter. However... microwave transformers have one end of the HV winding connected to the frame (ground) of the transformer so that wire will have to be carefully removed and properly insulated - probably not the best idea to use MOT's. If the 3200 you are mentioning above is DC out of a rectifier, that is a bit high even into a fully loaded choke input supply which will drop it to about 90% or 2900 volts. Be sure to use a bleeder resistor on your HV PS and be safe... High Voltages at this level should be considered, Death On Contact. You can find my email address on QRZ.com 73 WA4QGA
Not for nothing my friend but every 2 of amplifier or receiver that I've ever worked on. E I've always removed the tubes before I flipped it upside down. I found it a lot safer for the tubes if I did that because I made the same mistake. You're doing right now moving it around with the tubes still in it. And I've broken a couple of tubes doing it that very. It's not very safe for the tubes which you're moving it around like that
I am surprized, the caps you installed look like the cheep chinese chech & chong brand.they have been known to test good new and then short lived under use.
I think that happened to me twice as the voltage doubler capacitors failed twice. After the last replacements, a few years ago, they have been stable and reliable and run several hours per day.
As soon as you opened up the case and I saw the ChengX capacitors I knew that was going to be the problem. They are the WORST capacitors ever.
This is the third time I have had issues with the electrolytic capacitors in this unit. In this case, I think it was a little over-voltaged at start up but on two occasions a few years back, the capacitors in the power supply went bad - I believe opened. The HV power supply is a voltage doubler and nothing is over-voltage in that area - the two larger brown ones in the video. Maybe I changed brands and that is why they have lasted the last few years. Like you say - just junk capacitors. I have electrolytic capacitors in McIntosh amplifiers 50+ years old and they work perfectly. This little amplifier can run 12-15 hours a day at times as I have it on with background music. Thanks for your comments.
❗ Promo SM