Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 13, 2013, 03:55:31 pm
-
Hi guys, An Australian built vintage amp, that needs a bit TLC BUT I need help with HOW I can maintain its VINTAGENESS when the usual components are stuffed and the ORIGINALS are long gone. :think1:
I know that you guys are the BOMB when come to restorations so give'us ya best......
I have not poked it yet, as I have not found a schem so reverse engineering will have to be applied, any tips on doing this.....
THANKS
-
Looks interesting Timbo.
It's got verb and trem built in, is it all tube?
Brad :icon_biggrin:
-
Yeah, Isn't it a GEM :icon_biggrin:
-
Hey it might be a good sounding amp, who knows?
Brad :icon_biggrin:
-
Good score Timbo looks like a real jem my friend.
Bill
-
Hi guys, An Australian built vintage amp, that needs a bit TLC BUT I need help with HOW I can maintain its VINTAGENESS when the usual components are stuffed and the ORIGINALS are long gone. :think1:
I know that you guys are the BOMB when come to restorations so give'us ya best......
I have not poked it yet, as I have not found a schem so reverse engineering will have to be applied, any tips on doing this.....
THANKS
take lots of high res photos with several angles of each section with good light on the subject before you start, that way you have a record of anything that may be amiss before you start the restoration, also, we can all chip in, of course, only if you need us to help with reverse engineering and/or the restoration. ;-)
happy rosin smokin'
--pete
-
Thanks guys, Think I taken about 30 photos.
-
Got some OOOOZZZZ happening with the BIG Electrolytes.
-
What does the chassis exterior look like? Especially curious about the reverb unit.
Gonna need a big soldering gun (such as Weller D550) to replace those filter caps.
There's a mystery component block in the tone stack. Probably just a RC network.
The tremolo uses a lamp/LDR. I can't tell if the lamp is incandescent or neon. Does the lamp have two parallel rods inside (neon) or a filament (incandescent)? Since the lamp/LDR is exposed to light you'll need to turn off the lights for proper trem operation if the amp is on the bench as shown in pics.
Will you be restoring this for yourself? I think the guy that built it is still around and still building some amps. He may give you a schematic. Worth the effort to try to contact him.
-
There's a mystery component block in the tone stack. Probably just a RC network.
I think your right, it's sitting right behind and wired up to the tone controls.
Brad :icon_biggrin:
-
Hi guys, I had the day off yesterday and it took the hole day to reverse engineer it, it was a bit of a challenge.
Yes it has a mystery box that is filled with something that is fairly hard so it going to have to stay a MYSTERY, maybe someone could take a stab at its content.
Sluckey, I have done a schem to what I can see so far, as part of the reverb is hidden behind the tank (reverb transformer)i'll remove the chassis and tank today, the tank has been hard wired to the chassis.
Yes, its is a neon.Thanks
-
Very nice work Timbo. :icon_biggrin:
But, are you sure about V6, the LFO? It's cathode and by-pass cap do not go directly to ground. You show what would be their ground connection drawn into the plate to grid cap/resistor loop.
Yes it has a mystery box that is filled with something that is fairly hard so it going to have to stay a MYSTERY, maybe someone could take a stab at its content.
Like Sluckey said it's the TS RC network, what are the values in there? :dontknow:
He's using a reverb pot with 2 SPST switches on the back for turning on/off the dcv for the reverb?
Looks like a bias balance for the PI plate R's?
Brad :think1:
-
Phil Dreoni was the owner of Fisonic in the 1960's. You should still be able to contact him.
http://au.linkedin.com/pub/philip-dreoni/64/b35/73b (http://au.linkedin.com/pub/philip-dreoni/64/b35/73b)
-
Thanks guys, It was good fun trying to follow the maze.
Thanks darryl. :icon_biggrin:
-
Hi guys, Did some more checking and found a couple of GOOFs.
Other than the mystery box the only thing that I don't know is the thing on the back of the reverb pot? and does not appear to be a switch. Thanks
-
Hi guys, Did some more checking and found a couple of GOOFs.
Other than the mystery box the only thing that I don't know is the thing on the back of the reverb pot? and does not appear to be a switch. Thanks
push-pull switch? SPDT?
-
DL, If it is a switch then its lost its "click". So if the opposing lugs are connected then this would power the plates of the each tube and the reverb could be "turned off" and the foot sw. just to turn it off on the fly :dontknow:
I have taken readings of the component values to see how they have faired over the past 48yrs. Thanks
-
Hi guys, I can confirm that it is a rotary Sw. and that the alloy part inside is worn and lost its CLICK. I removed a pot/switch from one of the old amps I bought a while back and found replacement that I can change over the switch but stamped on the back has it as 250vAC@2A or 12vDC@10A will this do the trick.Thanks
-
> has it as 250vAC@2A
Will probably "lose its click" in the year 2053, just like the one before it.
They are all the same except number of contacts.
Using it in a ~~300V circuit with a many uFd cap does eventually burn the contact.
I'd go with it. If you or your kid or a new owner is still using it in the 2050s, worry about it then again.
-
Thanks PRR, I tried to graft some bits together but it did not work,so I played around with the original and managed to retrieve the click. Only time will tell if it holds. :icon_biggrin:
A local lad has also acquired one as well, and past on this info http://dalmura.com.au/projects/Fisonic%2030WGR%20info.pdf (http://dalmura.com.au/projects/Fisonic%2030WGR%20info.pdf) His is a bit different and has possible problems on startup. Thanks
{EDIT: un-munged URL to PDF -PRR}
-
Links not working.
Brad :dontknow:
-
Links not working.
Try this: http://dalmura.com.au/projects/Fisonic%2030WGR%20photos.pdf (http://dalmura.com.au/projects/Fisonic%2030WGR%20photos.pdf)
-
Thanks Darryl, that works.
Brad :icon_biggrin:
-
Hi guys, Just waiting on some parts, so a few more pics.........
-
Hi guys, had a bit of a set back. I couldn't find any specs on the PT so I fired it up (no tubes in) to measure the HT and got a small hum and after a few seconds got the magic smoke :sad2: but blew no fuses.So I removed the PT from chassis and cracked it open and its TOAST and stinks like burnt toast. Can this be rewound or is a NEWY in order.Thanks
-
TIMBO,
I have taken transformers to local transformer rewinding services with great success in the past.
(Google is your friend)
Don't start unwinding it or they might lose info on how to do it.
All the best.
-
Hey mate , I just connected the primary up and I am getting nothing on the HT or heaters. :cussing:
-
That's undesirable, generally fatal.
Check DC resistance of primary for open circuit? Unlikely that it has an internal fuse.
If the iron is in reasonable condition a rewind used to be a lot cheaper than a new transformer.
Ceriatone prices are pretty good, item and freight, for our region - if they have an equivalent.
Their Plexi 50 Power Transformer looks close. PRI 100 - 120 -240. SEC 325-0-325. Heaters 3.15-0-3.15 (6.3 CT). RECT FIL 0-5.
Search Engine "Transformer rewire CITY STATE", "Transformer services CITY STATE" etc etc.
How's the STC coming along.
All the best.
-
The STC been put hold for the moment as I was keen to here how this sounded.
Dougs bassman PT replacement sounds like a good match just don't know how much this amp will draw
-
I thought Doug did not stock 240 V mains transformers. :dontknow:
Current draw shouldn't be much different to any other 2 X 6L6 + 4 X 12A_7 + 1 X Tube Rectifier amp really.
Allow for a 50 Watter and you've got room to spare. 40 Watt minimum. ( I like to over engineer stuff a bit )
-
Thanks Glennjeff, I was looking for something that had 325-0-325 on the HT but i'm not sure how the calculate the total current draw for all the tubes to give me an idea of what I need. :icon_biggrin:
-
TIMBO,
Calculating the current and then allowing an appropriate amount of spare capacity is a real work of engineering and mathamatics. I could do it, but would have to drag a boxfull of old Uni texts out of the shed and spend several days working on it. Its one of those areas where having some experience and good "rules of thumb" may be preferred unless your going into commercial production IMHO.
Example of a "rule of thumb" approach:
Each 6L6 will be biased at about 50mA for HT (known from experience and the approximate HT voltage 325-0-325 will give)
Each 6L6 will draw 900mA heater current (from datasheet)
Each 12A_7 will need about between 1 to 5 mA depending on tube type and how it is biased (draw load lines on transfer characteristic).
With 12AX7 allow for 3 mA for each triode, which is the max they can handle at this voltage approx. 350 V. So that's 6mA per tube
Each 12AX7 will draw 300mA heater current. (from datasheet)
6AV6 is about the same as half a 12AX7 except the diodes are not being used in this case so allow 3mA HT for each triode. 3mA per tube.
The heater current for each 6AV6 tube is specified at 300mA, so allow 300 mA heater current for each 6AV6 tube.
6BM8. From a look at the datasheet it looks like the pentode part of the tube can deliver a bit of power as the quoted max anode current is 35mA for the pentode and 3.5mA for the triode, so allow 38.5mA HT current for each 6BM8.
The datasheet for the 6BM8 lists heater current at 780mA, allow that much.
The 6CA4 requires 1 Amp of heater current at 6.3 V. Now do some math to get a ballpark figure.
VALVE HT Current Heater current
3 X 12 AX7 18 mA 900 mA
2 X 6AV6 6mA 600 mA
1 X 6BM8 38.5 mA 780 mA
2 X 6L6 100mA 1800 mA
1 X 6CA4 N/A 1000 mA
TOTAL(approx) 165 mA 5.3 Amp
A typical 60's vintage 50W fender transformer replacement is around 125mA HT current and 3A heater current (6.3V) as they have a separate 5V winding for rectifier heaters, in your case you need a more robust 6.3 winding because the rectifier runs off 6.3 volts.
Conclusion: Although we have used some high value assumptions to come to this conclusion, A 50 Watt transformer may not have sufficient HT or heater current for your application. Check the transformer specs carefully and choose an appropriately robust one.
THAT WAS A RULE OF THUMB EXAMPLE.
I have an Asian supplied transformer for a 35W trainwreck style amp that is rated at 300mA HT and 7A heater, tons of spare capacity, so check the specs from your supplier.
-
> i'm not sure how the calculate the total current draw for all the tubes to give me an idea of what I need
Every scrap of info you need is IN Tim's PDF in TIMBO's post.
> good "rules of thumb" may be preferred
Your "50mA/6L6" may be way-low. A FIX-bias amp may idle at 50mA, but will suck MUCH more at FULL roar.
The databooks are your friends. IIRC a pair 6L6GC with a certain B+ and load will suck over 210mA. OTOH a *cathode*-biased team can't draw much more than 2*30= 60 Watts/pair, idle or roaring. Pre-6L6GC types not even that much.
And in almost any audio amp, the POWER bottles suck 90% of the current. So it is hardly worth summing-up all the little ones, just round-up the big ones.
Tim's PDF says (page 5) 350V at 120mA to 335V at 171mA.
Also documents his PT as 325-0-325V with 80 ohms each side, a dozen ohms on the wall-side. The ohms is stuff we need to know, but is hard to use, and the PT makers don't bother to tell us. I'd just find 325V rated for 150mA-200mA, and cross my fingers.
Also a heater winding with enough Amps. Quick-trick is to figure 1A for each 6L6, 1A for every three 12A_7s, and see what you find. 4 Amps, not-quite. Although in this case you *also* have several "odd" types and one is a heater-pig, so you should re-check Glennjeff's math.
-
Just to be clear.
What I did there was an example of one possible way to approach the problem, NOT A SOLUTION TO THE PROBLEM, even if somewhat relevant to the problem.
If as PRR states, the 6L6's are going to draw 210mA or more (I think he would know) then you should up the HT current rating to 265mA, IN MY EXAMPLE.
I managed to miss that PDF. I note that it states heater current as 5.1 A so my guestimate of 5.3 amps was rather accurate, and conservative. One could probably find the HT current in that document if doing due diligence on a restoration.
All the best.
-
Thanks for doing the hard stuff for us guys, I found that there is some difference between the two amps so its a bit hard to compare.
So going off the info you guys have supplied, leads me to thinking how the PT has come to its CHARD demise.
When I first got the amp I took pictures of everything but when I removed the rect tube I also removed what was left of the tubes markings :BangHead: so there been a bit of conjecture to what it would be and with the little knowledge I have thought that it was a 6CA4 being rated @ 150mAs
So if this being the case then would the 6AC4 that is looking slightly chard itself has given up the ghost and taken out the PT?????
......just find 325v rated for 150mA-200mA, and cross your fingers :l2: That's a good one, but I thought that I was the only one that had my fingers crossed when I flicked the switch.
Sourcing a new PT is not a problem but finding that the 6CA4 isn't going to cut the mustard could be a problem OR is there another 9pin rect tube rated higher than the 6ac4????
Thanks for your help guys.
-
TIMBO,
The 6CA4 is still in production by JJ, which is good if you want to maintain originality and serviceability.
It is rated at 150mA constant load, not that much different to a GZ34 @ 165mA. There is a lot of headroom though for peak current and the filter caps smooth out the load. It just might be OK to cross your fingers and hope on that one, unless you plan on using it for flat at, always saturated metal, or something. :rolleyes: :dontknow: :icon_biggrin:
After doing an extensive search on the web: 9 pin dual anode rectifiers are uncommon and the 6CA4 is the beefiest one.
-
I was going to replace it with one of these http://www.svetlana.com/pdf/electro-harmonix/6ca4eh.pdf (http://www.svetlana.com/pdf/electro-harmonix/6ca4eh.pdf) as I have one on hand.
-
Cool, it's definitely still in production then. Have had no problems with EH's valves, and a lot of them are stocked in my local audio geek shop. Not the 6CA4 though.
Had an enquiry about restoring some old Oz Goldentones recently, that'll be some fun nostalgia.
-
6CA4 is the same as EZ81. Sounds kinda light weight for use with 6L6s.
-
slucky,
What would one do, faithful restoration or correct an obvious 50 year old bit of bad engineering.
I'd put an octal in it for sure, TIMBO seems like he wants to keep it's antique value.
I think they must have played rather quietly back in the day, and probably thought it was radical loud.
-
This is something that LooseChange and I put together yrs and yrs ago & I still reference it when designing an amp and figuring out PT current and volts.
Tube Data Cheat Sheet
With respect, Tubenit
-
unless that amp has serious collector value, then upgrade it, esp. if it's going to be used. bear in mind though there is already a reduction in collector value due to fried PT.
rectifying poor engineering (pun intended) has no time limit. @ 400V the EZ81 is rated @ 125mA. the GZ34 is rated @ 250mA.
it's a cathode biased super reverb, so try to keep the B+ around 400V with 375V or so to the screens.
i'm guessing the mystery module is a bax/james tone stack circuit pack?
you're in 50Hz mains country correct? OZ? PSU designer shows hammond 372HX is potentially a good match for slightly higher B+ the 367X is a good match. i used PSU designerII with a GZ34.
--pete
-
Hi guys, Its a bit of a bummer that its not as good as a find as I hoped but its still a little gem. As for restoring it to its former glory and originality is not achievable due to lack of vintage parts and finishers.
If the only way to restore it is to use modern parts then that is all I can do and at the cost of replacing the PT I think it would be dumb to let some bad engineering fry a good PT. I have a 5U4GB on hand and if this will be a better replacement then I think that it will at least get it fired up safely and not alter the overall sound of the amp.
As a side note, the amp never had a mains fuse, so right from the start it was a POOR design and fixing those bad bits can only make it better.
I mention finishers, the tolex is in bad shape and the speaker box has no grill cloth, I searched for a replacement but nothing turned up. This is another thing that knocks down the vintage value.
I have the CHEAT SHEET and that was what sparked this whole topic, its a very useful item...... With that in mind it would be good to maybe refine it or add some of the oddball tubes that we use.....
I posted on my local forum and one of the guys "hit the nail on the head" with his reply, IF it was the only one of its kind in the world then it should be put in a glass case and admired by all but it is a tool that should be enjoyed by all and if the original builder could see his creation being used after 50years and maybe another 50years I think it would make him proud.
This amp has no footprint to give me any expectations of sound and engineering but its IS a bit of old aussie stuff and will probably never venture outside my bedroom/mancave.Thanks
Than
-
Hi guys, Found this as a good replacement http://www.classictone.net/40-18096.pdf (http://www.classictone.net/40-18096.pdf) but of course I have to cut a hole to suit :BangHead: OR this http://www.classictone.net/40-18094.pdf (http://www.classictone.net/40-18094.pdf) at a higher HT to give me options on the HT and I don't have to cut the chassis :think1: Thanks
-
Hi guys, Took a look at the speaker cab and its a little worst for wear, so I will recover it in new tolex. The baffle board is not great so I will replace it with 3/4" ply and new grill cloth,the backboard will be replaced with 1/2" ply
Had a win with the speakers, look to be in good condition and the info, have then to be 50W and 15ohms with a nominal resonance of 65Hz. Measured DC resistance of 14.2ohms per speaker. :icon_biggrin:
-
The one that does not fit looks more able to handle the current requirements, :BangHead:
The one that has the correct voltage and fits looks a bit whimpy. :cussing:
The factory that made those Plessey 12UX50 speakers is still operating in Melbourne. It is now owned by the person that was chief engineer in the seventies. You may even be able to get re-cone kits for those speakers, had three 12U50's in the first amp I ever built, they sounded like a million bucks. Now called Lorantz Speakers, and they make a great Greenback clone which is exported to the USA for use in boutique amps.
-
Hi guys, Amusing my self I took a look at the tubes that came with the amp and I was a bit surprised at what I found.The 12AX7s all Mullard made in Australia, date code 005, the 6av6s Mullard made in Australia, date code 325 and 6bm8 Mullard as well.
Now the two 6L6s, the one on the right is an RCA USA "Black Plate" with side getter and the left is a Sylvania/Phillips double top getter both were made between 1960-70 as per this site https://www.tubeworld.com/6l6gc.htm#6L6GCRCA (https://www.tubeworld.com/6l6gc.htm#6L6GCRCA) and looks like there worth a few bucks :dontknow:
This leads me to the next question, these were in the amp when I got it but I would think that a matched pair of 6L6s would have been in it when new. :think1:
The balance pot in the circuit after the inverter is new to me and I have no idea of its use, could this be used to match mismatched tubes :w2: Thanks
-
This leads me to the next question, these were in the amp when I got it but I would think that a matched pair of 6L6s would have been in it when new. :think1:
The balance pot in the circuit after the inverter is new to me and I have no idea of its use, could this be used to match mismatched tubes :w2: Thanks
When new, both tubes were likely of the same make, but itīs unlikely that they would install matched pair in the amp when new. The balance pot is used to adjust the AC balance of the PI (by changing the Raīs of the LTP).
-
It's unlikely the factory installed a fat and a tall.
Normally they got a crate of 144 tubes (all alike (not matched)) and each amp got two tubes from the crate.
So the original 6L6 were replaced at some point. Power tubes get the most abuse; no surprise that one or both failed or weakened or the owner just got dubious and replaced them. The fat and tall types are nominally "equivalent" so it should work. Anyway you can't be too fussy at 7pm on Saturday night and if it sounds OK, who cares how it looks?
Set the balance pot smack in the middle. Or replace it with two fixed resistors. In High-Fidelity it makes sense to allow for 20% variation, adjusted by the THD meter. It does not make sense to have a 100% adjustment, nor is lowest-THD our goal in rock-n-roll.
-
Thanks guys, I do not have a tube tester to see how close they are as a match, is there any other way to measure them???
-
Read the last two paragraphs here (http://www.el34world.com/charts/fenderservice5.htm). To balance them, you would also need to use two separate cathode resistors/capacitors, not sure if it is worth the trouble since they will drift off balance as the tubes age anyway. Some imbalance contributes a bit more distortion which it is not necessary a bad thing in a guitar amp (within reason of course).
-
Thanks jazbo, I was thinking of doing that as I do it to all of my other builds. I do have a match pair of 6L6s but was curious of the effect that a mismatch would have. Thanks
-
Hi guys, Transformer has arrived and its a great fit to the old bell ends. The newy is a laydown type with all the wires on one side.New specs are 325-0-325@180mAs,5v@3A and 6.3v@4.5A which are similar to the original. :icon_biggrin:
-
Hi guys, transformer is in and fired up :wav: Checked voltages and all look good. With nothing plugged in just a faint hum even when volumes are turned up.
When the reverb is turned OFF (not via foot sw) the voltages at each node is increased by 15v the readings are :-
REVERB OFF
NODE A - 380v
B - 353v
C - 294v
D - 255v
E - 239v
These are about what they should be with a 5U4GB rect
With these voltages I calculate the power output to be about 32w
Please check my calcs..
Plate - 372v
Screen - 351v
Cathode - 28.9v
Across 1ohm res. 52.6mA
Between Plate and cathode - 348v
I'm just a bit concerned that the OT is rated at 25w and as this is built to a minimum standard that it might do damage.
So far so good :icon_biggrin:
-
The so far so good has gotten even better :worthy1:. This amp sounds fantastic. Not a hint of BUZZ or HUM. Had a small issue with one of the volume pots being a bit scratchy and intermittent so I pulled it apart for a clean and bobs your uncle, just still a little scratchy now. Also the input jacks had corrosion inside so they were replaced and alls good. I also changed two of the grid resistors on the inputs to 33k, just for the effect.
I was concerned that it may be putting out too many watts for the OT having calculated 18w per tube and having a 25w OT but after playing awhile the OT was warm but some of that was coming from the 6L6s
As I have posted the reverb tank I think is dead and not worth trying to fix as tanks are not very expensive and a replacement is a 4FB2A1C with an input of 1475ohms and an output of 2250ohms, vertical mount as there is no room for a horizontal tank.
My only real concern is the tube sockets are very loose (octals were OK)and after pushing the grips( the things inside that grip onto the tube pins)together with the point of a probe they were a little better, I even tried to spread the tube pins a bit and this also helped but not as good as I had hoped. The option of replacing the old sockets is not one that I would look forward too, any quick fixes for this.
As said, to me the amp sounds VINTAGE with a very articulate crisp sound with nice sustain and when cranked can do a rock chord with out any problems.
The two channels have there own sound one brite and crispy and the other a little dark but with a warm feel (this one suits the blues,YUMMY)
I think the speaker cab suits the head very well, no bass buzzes or rattles is also a little dark but can hit the highs, also YUMMY
It always seems that any of these old amps that have vibrato have there problems and don't work, I was lucky with this one and did not have to touch a thing.
To me the amp is working fine and when the CC resistor have drifted as much as some have maybe this has added some of that VINTAGE MOJO :think1: and at this stage I won't be changing any to the original value, unless anyone can see a particular resistor that may need to be replaced for safety and avoiding any damage to the circuit Thanks
-
As I have posted the reverb tank I think is dead and not worth trying to fix as tanks are not very expensive and a replacement is a 4FB2A1C with an input of 1475ohms and an output of 2250ohms, vertical mount as there is no room for a horizontal tank.
Since your tank is transformer driven I'm betting the input is 8Ω. See if you can get your hands on a 4AB3C1B typical Fender tank to patch in before you buy that high impedance 4FB2A1C.
You can check the input requirements by checking the dc resistance of the secondary of the driver transformer with the tank unplugged. If it is under 10Ω DCR then you don't need a high impedance input tank.
-
Thanks again Sluckey, Just measured is 70.2ohm
-
Hi guys, The new tank has arrived and is now fitted. Fitting it between the transom and the bead at the bottom gives better clearance between the tank and 6BM8 tube.
I like reverb a lot this has plenty, @1 just a slight hint and @10 it goes a little nuts, so @5 is a good amount so at this stage I don't think it needs to be tweaked.
Gave the cab some TLC and is looking great :icon_biggrin:
Tubes......
All preamp and PI are mullards and are sounding great.
Power tubes JJ6L6GC also sounding great.
DID someone mention KT66s, been hearing a lot about these and they look PRETTY cool but can't find much about cathode biased tubes.
What are a good value for money tubes and can they be cathode biased.Thanks
-
Since the 6BM8 has a triode and a pentode in the same envelope, you can't just plug in any old pentode/tetrode in there. So for the same pinout, you can use ECL82 or 6F3P (Chinese or Russian).
-
Hi guys, Got some new tubes in the mail, a pair of Tung Sol 6L6G and a GZ34. A new set of voltages taken and all looks great.
A small concern with the power sw., it sparks when turning from standby to ON. I'm sure in time this may burn the contacts and possible replacement of the sw. Is there any thing I can do to reduce this to avoid having to replace it.
DO I replace the old grille cloth on the head cab to match the speaker cab???????
Thanks to all for a great project. :icon_biggrin:
-
Some final pics.......
-
Some more...............
-
very nice work!
:icon_biggrin:
--pete
-
Hi guys, Had a chance to crank it up a bit and its not good :sad2:The volume pot on channel 1 is scratchy (worn) and this causes crackling and if not turned down to 0 when playing channel 2 also causes crackling. Channel 2 volume not too bad just a small spot that crackles, so a couple of new pots needed.
When playing notes on G,B and E its great, lots of tone and clarity and when turned up only just a hint of breakup and some sustain (this is good) but more would be better
But when playing notes on E,A and D lots of flabbiness and wont play power chords.
With this problem the vibration causes the valves to rattle as they are gripped by the retainers but are loose in the socket. I had feared that the sockets may have to be replaced as I can't be sure that the tension on the valve pins won't loosen in time.
I do not have another amp that I can compare this amp with or any original recordings so can anyone give me a comparison going by the schem to another type of amp.
I think I need to address these couple of problems. Thanks
-
Hi guys , got some time to tidy up a few unfinished jobs. I needed to replace the two volume pots as they were very scratchy and was causing a few problems.
This has made the amp come alive tone wise as well as volume.
With the old pots, I would have to get it to about 5 before I got any real volume, now by the time it hit 2 its more than loud enough for a bedroom. :icon_biggrin:
I also increased the cathode resistor on the power tubes to see if this might reduce the volume a bit but at 2 its still loud, I will leave it for the moment as it will also reduce the output and not over work the OT.
I still have a problem with the tube sockets being loose, so i'll have to think about it some more as I don't want to try and replace the sockets. Thanks everyone,its a real gem :icon_biggrin:
-
I needed to replace the two volume pots as they were very scratchy and was causing a few problems. This has made the amp come alive tone wise as well as volume.
That's interesting, not only volume but tone improvement, good catch.
its a real gem :icon_biggrin:
Alright then, you worked it all out and your efforts paid off and then some.
Brad :bravo1:
-
Thanks Brad, yeah the next thing is the sockets. I'm not able to get the same as what been used. so I'm at a loss at what to do. The only other option is to custom a retainer :think1:
-
Loose tubes sorted. This was what I thought was the best solution and its worked well.
I'm trying some 6V6s as the 6l6s are way too loud for my small room and I was hoping the get a bit earlier breakup at a lower volume.
I got a pair of GTs with 427v on the plates, 31v on the cathode, 400v plate to cathode pumping out about 26w. I think that this is a little too high for them so I might try a bigger cathode resistor. I got a pair of Tung Sols i'll try. This project is just about done. :icon_biggrin:
-
That looks good, nice and neat.
Brad :icon_biggrin: