Just finished my quickie build, just bread boarded it to see what it would do.
I did not want to go SE because I wanted a cheap output transformer that anyone could pick up. I wanted to try a 70 volt line transformer, a 120V power transformer, and a 12V transformer for the heaters. Did not do a cost breakdown yet but it should get under the $100. Sounded not too bad clean although without a efficient speaker, used a 8" paging speaker that would be used on a 70v system, the level is just barely adequate. Used a 12" which is around 97dB and it is livable. Turning things up both are loud enough if you do not mind some distortion.
Not sure how much farther I will go, do not really care to compete, just wanted to give it a try.
I did not want to go SE because I wanted a cheap output transformer that anyone could pick up. I wanted to try a 70 volt line transformer, a 120V power transformer, and a 12V transformer for the heaters. Did not do a cost breakdown yet but it should get under the $100. Sounded not too bad clean although without a efficient speaker, used a 8" paging speaker that would be used on a 70v system, the level is just barely adequate. Used a 12" which is around 97dB and it is livable. Turning things up both are loud enough if you do not mind some distortion.
Not sure how much farther I will go, do not really care to compete, just wanted to give it a try.
You might think about a bypass cap on the cathode resistor, usually on the 6AQ5's I use a 50uf/50V. This seems to reduce some of the raspy distortion and firm up the bass a good bit.
Fender used a 25UF on many of their cathode biased amps.
Remember, a bedroom amp will not be overly loud and distortion is generally sought after by the average player.
YMMV
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Look again... the cathode resistor isn't bypassed since it's a self-split design - it's the only coupling to the other half of the push pull output stage.
There have been a few questions about speakers and I wanted to mention Weber. We used two Signature 12Bs in my son's bigger amp and they worked very well for him:
https://taweber.powweb.com/weber/csig12.htm
There is no 10B but a 10S or 8S should work well in smaller amps:
https://taweber.powweb.com/weber/csig10s.htm
https://taweber.powweb.com/weber/
These are made to order in their USA shop last I checked.
They have amp and many other parts also.
https://taweber.powweb.com/weber/csig12.htm
There is no 10B but a 10S or 8S should work well in smaller amps:
https://taweber.powweb.com/weber/csig10s.htm
https://taweber.powweb.com/weber/
These are made to order in their USA shop last I checked.
They have amp and many other parts also.
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I used Weber in several amps a long time ago. They were a great speaker at a good price. I remember seeing something on the guitar amp forums a few years back about the company changing hands resulting in some delivery or quality problems. I don't remember the details since I haven't made any guitar amps in a while.
I have purchased several cheap speakers to play with. I will start a simple 1X8 and a 2X6 cabinet tonight in woodshop class. I can always try better speakers at a later date.
The owner Ted Weber died in 2009 but his family and son are keeping it going:
Rest Easy, Ted Weber
They are not able to ship fast since they have to put an order into the que to actually build them.
Rest Easy, Ted Weber
They are not able to ship fast since they have to put an order into the que to actually build them.
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After I posted I Googled and hit the guitar forums to find one of Ted's last posts:
I'm here, I'm breathing for the time being, and I'm answering emails as always. I only sleep about 3 hours a night, I pee a lot due to the drugs I'm on, so I spend a lot of time at the computer answering emails during the night since it is on the way to the bathroom.
As for my health, I have pulmonary sarcoidosis, a hardening of the lungs.
I'm on oxygen 24-7 and am waiting to be qualified for a transplant. Transplants are around 50% success rate at 5 years, so I'm not doing any long range planning.
Not to worry, though. If and when I take the trip, my son will probably replace me with a Hal 5000, and that will be a hell of a lot better for everyone.
Apparently the Hal 5000 took over and screwed things up for a while, now as you say the family runs the show and things look OK now. I should have bought one of his Alnico 8's instead of the Jensen I got. The Weber is cheaper and probably sounds better.
I'm here, I'm breathing for the time being, and I'm answering emails as always. I only sleep about 3 hours a night, I pee a lot due to the drugs I'm on, so I spend a lot of time at the computer answering emails during the night since it is on the way to the bathroom.
As for my health, I have pulmonary sarcoidosis, a hardening of the lungs.
I'm on oxygen 24-7 and am waiting to be qualified for a transplant. Transplants are around 50% success rate at 5 years, so I'm not doing any long range planning.
Not to worry, though. If and when I take the trip, my son will probably replace me with a Hal 5000, and that will be a hell of a lot better for everyone.
Apparently the Hal 5000 took over and screwed things up for a while, now as you say the family runs the show and things look OK now. I should have bought one of his Alnico 8's instead of the Jensen I got. The Weber is cheaper and probably sounds better.
My Peavey 8" guitar speaker and Peavey 8" Blue Marvel came in yesterday so I need to build a cabinet to test with unless they fit a generic cabinet I already have.
I didn't buy any to test from AES.
I think someone asked about Magnavox 12" console speakers, I also have a pair of 12" speakers from a Magnavox console, a pair of 12" speakers from a Webcor console, the G12M, and Webber VST Sig10F to test with.
Now to decide whether to stay with the SE 1W amp or change the output to PP and gof or 2-3W.
I didn't buy any to test from AES.
I think someone asked about Magnavox 12" console speakers, I also have a pair of 12" speakers from a Magnavox console, a pair of 12" speakers from a Webcor console, the G12M, and Webber VST Sig10F to test with.
Now to decide whether to stay with the SE 1W amp or change the output to PP and gof or 2-3W.
A bit more about Ted Weber, he was not very old - very sad:
Ted A. Weber Obituaries Kokomo Tribune; Kokomo, Indiana
I would like to give the person the option to play clean or dirty. With another volume control after the second triode the amount of grit can be dialed in and the volume can be played at whisper levels. Might not be output distortion, but at those levels I am not sure how much it matters.
Bumped up the voltage to 220V, like it better. With a 12" (12" ET65 - 65 Watts | Warehouse Guitar Speakers) it gave a good account for itself. With a 10" (10" Veteran - 20 Watts | Warehouse Guitar Speakers) a few dB quieter and a less thick, more delicate sound, like them both. The 8" also sound ok, a little smoother and quieter, maybe 5dB as compared to the 10".
Tried it with a VVR to knock down the volume, need to change some values to get it down below 90V, tone stayed more or less the same but I do not think it is worth the effort and the added cost as compared to a master volume. Probably more value in having separate bass and treble controls.
With the 12" and the 10" you can get fairly loud if you are into a real distorted sound. I need to bump up the grid stoppers a bit for playing with the controls pegged. Mind you my lead dress does leave much to be desired as you can tell. All in all I am impressed what you can do with a 4W 70V transformer.
What input Z does that 70V distribution OT present? I'm thinking of using a 6.3 V (about 8V unloaded) filament OT which has two 120V primaries which would present around 7K P-P.
Estimating the input Z for the 70V OT:
Watts = 10 = V*V/R so V secondary = SQRT(80) = 8.94 Vrms
Assuming 70 Vrms across each half to the primary. So N1/N2 = 140/8.94 = 15.66
So NxN = 245.23 and Zpri = 245.23 x 8 Ohms = 1962 Ohms P-P
(which seems a bit low for a 6AQ5, but putting that into Circlotron mode would get 4x 1962 = 7848 Ohms P-P )
Estimating the input Z for the 70V OT:
Watts = 10 = V*V/R so V secondary = SQRT(80) = 8.94 Vrms
Assuming 70 Vrms across each half to the primary. So N1/N2 = 140/8.94 = 15.66
So NxN = 245.23 and Zpri = 245.23 x 8 Ohms = 1962 Ohms P-P
(which seems a bit low for a 6AQ5, but putting that into Circlotron mode would get 4x 1962 = 7848 Ohms P-P )
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I looked at the transformer again, I was wrong, I am using a 10W rather than a 4W. I have a few and the funny thing is the core size is the same on them both.
Anyway the one I am using has taps for 2.5W, 1.25W and 0.625W. The ratio between the 2.5W and 1.25W is different than the 1.25W and 0.625W, I thought it would cause me some trouble but it does not seem to be. I measured the ratios but I can not find where I wrote them down now. For hifi the imbalance may be an issue but it seems close enough for R&R. I also have a 4 and 8 ohm tap so I can double or half my impedance. I also have a 8 and 16 ohm speaker that I am using. Not being one to sweat the details, I changed ratios on the tubes, going low and high. In the end it seemed any combination worked. I am in no danger of red plating the tubes so they seem to be taking what I throw at them. Not like I am trying to squeeze the last watt out of them anyway. I think you can get a slight change in sound but then again you also do with changing the voltage on the tubes. I also swapped out the 12AX7 for a 12AU7, less gain naturally, with a telecaster you can get a nice clean with enough breakup to make it interesting with guitar and volume controls on full.
I think the amp is more forgiving of the impedance since it is running in Class A rather than Class AB. Tried it in triode mode, still was able to rock out. Know that I know it has enough gain with a 12AU7 I think I will try a 12AU6 on the front end next. I am having more fun with this little guy than I thought I would. I have not even bothered to turn my scope on to see what really is happening. Maybe later.
Anyway the one I am using has taps for 2.5W, 1.25W and 0.625W. The ratio between the 2.5W and 1.25W is different than the 1.25W and 0.625W, I thought it would cause me some trouble but it does not seem to be. I measured the ratios but I can not find where I wrote them down now. For hifi the imbalance may be an issue but it seems close enough for R&R. I also have a 4 and 8 ohm tap so I can double or half my impedance. I also have a 8 and 16 ohm speaker that I am using. Not being one to sweat the details, I changed ratios on the tubes, going low and high. In the end it seemed any combination worked. I am in no danger of red plating the tubes so they seem to be taking what I throw at them. Not like I am trying to squeeze the last watt out of them anyway. I think you can get a slight change in sound but then again you also do with changing the voltage on the tubes. I also swapped out the 12AX7 for a 12AU7, less gain naturally, with a telecaster you can get a nice clean with enough breakup to make it interesting with guitar and volume controls on full.
I think the amp is more forgiving of the impedance since it is running in Class A rather than Class AB. Tried it in triode mode, still was able to rock out. Know that I know it has enough gain with a 12AU7 I think I will try a 12AU6 on the front end next. I am having more fun with this little guy than I thought I would. I have not even bothered to turn my scope on to see what really is happening. Maybe later.
You might want to follow this thread as most certainly there will be something fine that comes out of it.
I might point out that several years back 18W.com divided and a few newer very active amp forums were born.
Hosted by http://www.wattkins.com/forum
They have a sections called
sewatt.com which is everything single ended
ppwatt.com naturally push pull amps.
One of the more creative members developed a few low cost PCB's to convert the very cheap Epiphone Valve Jr amps into other classics.
When Terry Stinger passed(accidental), his wife with the help of the forum has continues to supply the pcb boards.
original site
GuitarAmplifierPCBs
You need to sign-up/join to view the forum site but as far as budget, simple to build, and easy to follow options, these are a great source as well.
They even set up a parts package deal for one model with a solid vendor.
Antique Electronic Supply
Instructions for valve Jr to 18W lite
I am able to drop down to cheap transistor radio volume (less than a watt)with loads of starved tube pre-amp distortion as well as nice cleans. The cleans tend to be a tad less dynamic & bright, but it amazes me that you can get that low with the VVR and remain fully functional.
edit;This is in an 18W PP cathode biased amp
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Speco T-7010?
I think the 70V transformer is meant to tap it's rated power from a 70VRMS audio signal when loaded with the specified speaker load. So the various primary taps are by my reckoning:
10W => (70^2) /10 => 490 ohms
2.5W => (70^2) / 2.5 => 1960 ohms
1.25W => 3920 ohms
0.625W => 7840 ohms
Cheers
The constant voltage system is based on 70.7 volts in the US 100 volts elsewhere when the electrical codes don't prohibit it. The idea is that power is voltage squared divided by resistance. So 70.7 squared is 5,000 and 100 is 10,000. It made the math easier. So 1 watt at 70 volts should be 5000 ohms. A 5 watt transformer would have the 5 watt tap as the center tap and the common and 1.25 watt taps as the ends of the coil. The more turns the higher the transformer ratio and the higher the impedance. The center tap will always be 1/4 of the highest impedance value. Of course that ignores winding losses.
Just bumped into this thread
I just found this thread. Really cool!!! Count me in for a challenge.
I recently built a Marshall 18W clone for my teenage daughter, and certainly have second thoughts about what I've done...
I got a couple ideas of as to how a "bedroom rockstar" amp should be built... And it just happened that this weekend is entirely free (family is away), so hopefully I will have something to show tonight.
Cheers, Paul
I just found this thread. Really cool!!! Count me in for a challenge.
I recently built a Marshall 18W clone for my teenage daughter, and certainly have second thoughts about what I've done...
I got a couple ideas of as to how a "bedroom rockstar" amp should be built... And it just happened that this weekend is entirely free (family is away), so hopefully I will have something to show tonight.
Cheers, Paul